# Benchmarking backup tools

# Test logs

## Introduction

This document keep trace of the tests performed that lead to the summarized results presented in the benchmark document. Several helper scripts detailed at the end of this document have been necessary to obtain these results.

## Testing Context

### Software under test

- dar version 2.7.0
- rsync version 3.1.3
- tar GNU tar 1.30 under Linux and GNU tar (gdar) 1.32 under FreeBSD

### Hardware used for testing

Performance tests has been performed on an HPE Proliant server (a ProLiant XL230a Gen9 running two Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz processors) running a Devuan beowulf Linux system. Other tests have been run from Virtual Machines (FreeBSD 12.1, Devuan 3.0.0) of a Proxmox hypervisor running one on an Intel Core i5-7400 (3 GHz) based computer.

## Test logs

### Completness of backup and restoration

#### Dar

`root@terre:/mnt/localdisk/Benchmark_tools# ./build_test_tree.bash SRC 1024+0 records in 1024+0 records out 1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00395381 s, 265 MB/s 1024+0 records in 1024+0 records out 1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00621889 s, 169 MB/s 1+0 records in 1+0 records out 1 byte copied, 0.000386102 s, 2.6 kB/s root@terre:/mnt/localdisk/Benchmark_tools# dar -c backup -R SRC -------------------------------------------- 14 inode(s) saved including 3 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 14 -------------------------------------------- EA saved for 1 inode(s) FSA saved for 5 inode(s) -------------------------------------------- root@terre:/mnt/localdisk/Benchmark_tools# mkdir DST root@terre:/mnt/localdisk/Benchmark_tools# dar -x backup -R DST -------------------------------------------- 14 inode(s) restored including 3 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 14 -------------------------------------------- EA restored for 1 inode(s) FSA restored for 1 inode(s) -------------------------------------------- root@terre:/mnt/localdisk/Benchmark_tools#`

We simply performed backup of `SRC` directory with dar's default options, then restore this backup into the `DST` directory, let's now compare `SRC` and `DST` contents:

`root@terre:/mnt/localdisk# du -s SRC DST 2068 SRC <e>1048</e> DST root@terre:/mnt/localdisk#`

The space used by `DST` is less than the space used by `SRC`! At first we could beleive that not all data could be restored, let's looking for the explanation:

`root@terre:/mnt/localdisk#ls -iRl SRC DST DST: total 1044 414844 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414850 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414848 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414842 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414841 prw-r--r-- 2 root root 0 Oct 28 11:09 pipe 414840 -rw-rwxr--+ 1 root root 1048576 Oct 28 11:09 plain_zeroed 414849 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random <e>414843 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file DST/SUB: total 4 414841 prw-r--r-- 2 root root 0 Oct 28 11:09 hard_linked_pipe 414846 srw-rw-rw- 2 root root 0 Oct 12 23:00 hard_linked_socket <e>414843 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414845 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414847 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random DST/dev: total 0 414846 srw-rw-rw- 2 root root 0 Oct 12 23:00 log SRC: total 2064 411386 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414836 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414835 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414834 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 pipe 414826 -rw-rwxr--+ 1 root root 1048576 Oct 28 11:09 plain_zeroed 414827 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file SRC/SUB: total 4 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 hard_linked_pipe 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 hard_linked_socket 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414830 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414831 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random SRC/dev: total 0 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 log root@terre:/mnt/localdisk#`

All files are present in `DST` and use the expected space usage, as reported by the `ls` command. We can also see that the hard linked inode were properly restored for plain file, named pipe and unix socket: the inode number in first column is the same (see colorized output above).

Maybe something is missing elsewhere?

`root@terre:/mnt/localdisk# getfacl SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- mask::rwx other::r-- # file: DST/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- mask::rwx other::r-- root@terre:/mnt/localdisk# getfattr -d SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed <e>user.hello="hello world!!!" # file: DST/plain_zeroed <e>user.hello="hello world!!!" root@terre:/mnt/localdisk/Benchmark_tools# lsattr SRC/plain_zeroed DST/plain_zeroed <e>s---i-d-------e---- SRC/plain_zeroed <e>s---i-d-------e---- DST/plain_zeroed root@terre:/mnt/localdisk/Benchmark_tools#`

To summarize:

- user and group ownership are restored
- permission are set correctly
- ACL are properly restored
- Extended Attributes also
- file system specific attributes are too
- hard links are restored

So what? Let's rerun `du` file by file:

`root@terre:/mnt/localdisk/Benchmark_tools# du -B1 SRC/* DST/* 8192 SRC/SUB 4096 SRC/dev 0 SRC/fd1 0 SRC/null <e>1048576 SRC/plain_zeroed 1052672 SRC/random 8192 DST/SUB 4096 DST/dev 0 DST/fd1 0 DST/null <e>4096 DST/plain_zeroed 1052672 DST/random root@terre:/mnt/localdisk/Benchmark_tools# ls -l SRC/plain_zeroed DST/plain_zeroed -rw-rwxr--+ 1 root root <e>1048576 Oct 21 18:40 DST/plain_zeroed -rw-rwxr--+ 1 root root <e>1048576 Oct 21 18:40 SRC/plain_zeroed root@terre:/mnt/localdisk/Benchmark_tools#`

OK here is the explanation: `plain_zeroed` file was using 1048576 bytes of disk space in SRC and consumes only 4096 bytes in DST, but it has its file size is still officially 1048576, it has thus become now a sparse file (not all zeroed bytes are stored).

`root@terre:/mnt/localdisk/Benchmark_tools# diff -s SRC/plain_zeroed DST/plain_zeroed Files SRC/plain_zeroed and DST/plain_zeroed are <e>identical root@terre:/mnt/localdisk/Benchmark_tools#`

But nothing changes from the user point of view, the restoration process with dar just optimized the space usage.

Let's continue checking the inode dates. As you know, Unix inode have several dates:

- atime, the access time, gives the last time the file's data has been accessed (read)
- mtime, the modification time, gives the last time the file's data has been modified (wrote)
- ctime, the change time, gives the last time the file's metadata in other word the inode properties (ownership, ACL, permissions, dates, ...) have been modified
- btime, the birth time or yet creation time, gives the time the file has been created on the current filesystem, this date is not present on all Unix system

The `ls -iRl` command we used so far does only show the mtime date moreover with a time accuracy of only one minute, while modern systems provide nanosecond precision. For that reason we will use the `stat` command instead to have all dates at the system time accuracy:

`root@terre:/mnt/localdisk/Benchmark_tools# stat SRC/random DST/random File: SRC/random Size: 1048576 Blocks: 2048 IO Block: 4096 regular file Device: 802h/2050d Inode: 414840 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) <e>Access: 2020-10-22 12:13:01.813319506 +0200 <e>Modify: 2020-10-22 12:12:57.765328555 +0200 Change: 2020-10-22 12:12:59.805323991 +0200 Birth: - File: DST/random Size: 1048576 Blocks: 2048 IO Block: 4096 regular file Device: 802h/2050d Inode: 414889 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) <e>Access: 2020-10-22 12:13:01.813319506 +0200 <e>Modify: 2020-10-22 12:12:57.765328555 +0200 Change: 2020-10-22 12:14:34.877131738 +0200 Birth: - root@terre:/mnt/localdisk/Benchmark_tools#`

From the above output we see that:

- atime is restored
- mtime is restored
- ctime is not restored

As we targeted this benchmark mainly for Linux which has not yet the `btime` available (Well some Linux file systems support btime but its access is not yet fully available to applications), we will thus momentarily change to a BSD system to play with `btime`. BSD systems include MACOS X, FreeBSD, NetBSD, butterflyBSD,... we will use FreeBSD here. Under FreeBSD, the `stat` command is not as easy to read as under Linux, however it is very flexible which we will leverage to mimic the Linux output:

`root@FreeBSD:~denis # which mystat mystat: aliased to <e>stat -f "%N%nAccess: %Sa%nModify: %Sm%nChange: %Sc%nBirth: %SB%n" !* root@FreeBSD:~denis # mystat SRC/random SRC/random Access: Oct 27 13:28:41 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 22 15:34:09 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:~denis # dar -c backup -R SRC -q root@FreeBSD:~denis # mkdir DST root@FreeBSD:~denis # dar -x backup -R DST -q root@FreeBSD:~denis # mystat DST/random DST/random Access: Oct 27 13:28:41 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 27 13:31:50 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:~denis #`

In conclusion dar also saves and restores `btime` properly.

#### Rsync

Let's do the same we did previously using rsync. We start by copying `SRC` directory to `DST`:

`root@terre:/mnt/localdisk/Benchmark_tools# chattr -i DST/plain_zeroed root@terre:/mnt/localdisk/Benchmark_tools# rm -rf DST root@terre:/mnt/localdisk/Benchmark_tools# mkdir DST root@terre:/mnt/localdisk/Benchmark_tools# rsync -arvHAXS SRC/* DST sending incremental file list created directory DST fd1 null pipe plain_zeroed random SUB/ SUB/hard_linked_pipe => pipe SUB/hard_linked_socket SUB/hard_linked_sparse_file SUB/symlink-broken -> random SUB/symlink-valid -> ../random dev/ dev/log => SUB/hard_linked_socket sparse_file => SUB/hard_linked_sparse_file sent 12,340,852 bytes received 198 bytes 24,682,100.00 bytes/sec total size is 22,577,173 speedup is 1.83 root@terre:/mnt/localdisk/Benchmark_tools#`

First note, the backup and restoration is done in one step, where dar was decorelating the backup operation from the restoration operation. The resulting backup needs not software to be restored (`DST` is a copy of `SRC`). For dar to reach the same result (without using storage for the backup) this implies two dar commands: `dar -c - -R SRC | dar -x - --sequential-read -R DST`. The situation is similar with `tar`, you need two commands to perform the same task: `tar -cf - | tar -xf -`

`root@terre:/mnt/localdisk/Benchmark_tools# du -s SRC DST 2056 SRC <e>1028 DST root@terre:/mnt/localdisk/Benchmark_tools#`

Here too, the restored data uses less space than the original data, sparse file have been taken into account (need specifying -S option) and space optimization of non sparse file is performed.

`root@terre:/mnt/localdisk/Benchmark_tools# ls -iRl SRC DST DST: total 12060 414843 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414844 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414840 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414841 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414842 prw-r--r-- 2 root root 0 Oct 28 11:09 pipe 414848 -rw-rwxr--+ 1 root root 1048576 Oct 28 11:09 plain_zeroed 414849 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random <e>414850 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file DST/SUB: total 10000 414842 prw-r--r-- 2 root root 0 Oct 28 11:09 hard_linked_pipe 414845 srw-rw-rw- 2 root root 0 Oct 12 23:00 hard_linked_socket <e>414850 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414846 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414847 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random DST/dev: total 0 414845 srw-rw-rw- 2 root root 0 Oct 12 23:00 log SRC: total 2064 411386 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414836 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414835 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414834 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 pipe 414826 -rw-rwxr--+ 1 root root 1048576 Oct 28 11:09 plain_zeroed 414827 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file SRC/SUB: total 4 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 hard_linked_pipe 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 hard_linked_socket 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414830 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414831 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random SRC/dev: total 0 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 log root@terre:/mnt/localdisk/Benchmark_tools#`

All files are present in `DST` and use the expected space usage, as reported by the `ls`. We can also see that all three hard linked inode (plain file, socket and named pipe) are restored properly. So we can suspect the cause of the size difference to be linked with sparse files:

Let's now check file's metadata:

`root@terre:/mnt/localdisk/Benchmark_tools# getfacl SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- mask::rwx other::r-- # file: DST/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- group::rwx other::r-- root@terre:/mnt/localdisk/Benchmark_tools# getfattr -d SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed <e>user.hello="hello world!!!" # file: DST/plain_zeroed <e>user.hello="hello world!!!"
root@terre:/mnt/localdisk/Benchmark_tools# lsattr SRC/plain_zeroed DST/plain_zeroed s---i-d-------e---- SRC/plain_zeroed --------------e---- DST/plain_zeroed root@terre:/mnt/localdisk/Benchmark_tools#stat SRC/random DST/random File: SRC/random Size: 1048582 Blocks: 2056 IO Block: 4096 regular file Device: 802h/2050d Inode: 414827 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) Access: 2020-10-28 11:09:59.977926733 +0100 <e>Modify: 2020-10-28 11:09:57.973931318 +0100 Change: 2020-10-28 11:09:57.973931318 +0100 Birth: - File: DST/random Size: 1048582 Blocks: 2056 IO Block: 4096 regular file Device: 802h/2050d Inode: 414849 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) Access: 2020-10-28 12:07:53.622841733 +0100 <e>Modify: 2020-10-28 11:09:57.973931318 +0100 Change: 2020-10-28 12:07:53.622841733 +0100 Birth: - root@terre:/mnt/localdisk/Benchmark_tools#`

So in summary:

- Permission are restored,
- user and group ownership are restored too,
- mtime is restored,
- File ACL are restored,
- Extended Attributes are restored

But

- filesystem specific attributes are not restored,
- atime is not restored,
- ctime is not restored

For btime as we did before, let's test under a FreeBSD system:

`root@FreeBSD:~denis # rm -rf DST root@FreeBSD:/home/denis # which mystat mystat: aliased to stat -f "%N%nAccess: %Sa%nModify: %Sm%nChange: %Sc%nBirth: %SB%n" !* root@FreeBSD:/home/denis # mystat SRC/random SRC/random Access: Oct 27 14:27:59 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 22 15:34:09 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:/home/denis # mkdir DST root@FreeBSD:/home/denis # rsync -arv SRC/* DST sending incremental file list fd1 null pipe plain_zeroed random sparse_file SUB/ SUB/hard_linked_socket SUB/hard_linked_sparse_file SUB/symlink-broken -> random SUB/symlink-valid -> ../random dev/ dev/log -> /var/run/log sent 22,583,283 bytes received 129 bytes 45,166,824.00 bytes/sec total size is 22,577,179 speedup is 1.00 root@FreeBSD:/home/denis # mystat DST/random DST/random Access: Oct 27 14:28:53 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 27 14:28:53 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:/home/denis #`

So, birthtime is properly restored.

#### Tar

As done with previously, let's save and restore the `SRC` directory to `DST`... Note that by default no sparse file is taken into account (this is why we added the `-S option`), same with acl (so we added the `--acl` option) and Extended Attributes (unless `--xattrs` is added). The tar command-line becomes thus a bit longer:

`root@terre:/mnt/localdisk/Benchmark_tools# rm -rf DST root@terre:/mnt/localdisk/Benchmark_tools# cd SRC root@terre:/mnt/localdisk/Benchmark_tools/SRC# tar --acl --xattrs -cSf ../backup.tar * tar: SUB/hard_linked_socket: socket ignored tar: dev/log: socket ignored root@terre:/mnt/localdisk/Benchmark_tools/SRC# cd ../ root@terre:/mnt/localdisk/Benchmark_tools# mkdir DST root@terre:/mnt/localdisk/Benchmark_tools# cd DST root@terre:/mnt/localdisk/Benchmark_tools/DST# tar --acl --xattrs -xSf ../backup.tar root@terre:/mnt/localdisk/Benchmark_tools/DST# cd .. root@terre:/mnt/localdisk/Benchmark_tools#`

Now let's compare the restored data with the original:

`root@terre:/mnt/localdisk/Benchmark_tools# du -s SRC DST 2068 SRC <e>2068 DST root@terre:/mnt/localdisk/Benchmark_tools#`

The sparse file has been properly restored (thanks to the `-S option` for that) but not space optimization has been performed.

`root@terre:/mnt/localdisk/Benchmark_tools# ls -iRl SRC DST DST: total 12060 414841 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414846 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414847 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414848 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414849 prw-r--r-- 1 root root 0 Oct 28 11:09 pipe 414850 -rw-rwxr-- 1 root root 1048576 Oct 28 11:09 plain_zeroed 414852 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random <e>414843 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file DST/SUB: total 10000 414845 prw-r--r-- 1 root root 0 Oct 28 11:09 hard_linked_pipe <e>414843 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414842 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414844 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random DST/dev: total 0 SRC: total 2064 411386 drwxr-xr-x 2 root root 4096 Oct 28 11:09 SUB 414836 drwxr-xr-x 2 root root 4096 Oct 22 11:09 dev 414835 brw-r--r-- 1 root root 2, 1 Oct 28 11:09 fd1 414834 crw-r--r-- 1 root root 3, 1 Oct 28 11:09 null 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 pipe 414826 -rw-rwxr--+ 1 root root 1048576 Oct 28 11:09 plain_zeroed 414827 -rw-r--r-- 1 nobody root 1048582 Oct 28 11:09 random 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 sparse_file SRC/SUB: total 4 414832 prw-r--r-- 2 root root 0 Oct 28 11:09 hard_linked_pipe 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 hard_linked_socket 414828 -rw-r--r-- 2 root root 10240000 Oct 28 11:09 hard_linked_sparse_file 414830 lrwxrwxrwx 1 root root 6 Oct 28 11:09 symlink-broken -> random 414831 lrwxrwxrwx 1 bin daemon 9 Oct 28 11:09 symlink-valid -> ../random SRC/dev: total 0 414837 srw-rw-rw- 2 root root 0 Oct 12 23:00 log root@terre:/mnt/localdisk/Benchmark_tools#`

The warning was not vain, `SUB/hard_linked_socket` and `log` are missing in `DST`. This is however a minor problem as usually unix sockets get recreated by the process using them. However we might have some permission and ownership to set back, by hand. A possible use case is `syslog` daemon, when let available for a chrooted process or container (MTA, or other network service).

The second problem is a bit more annoying: the hard linked fifo (aka named pipe) is silently restored as two independent named pipes (the inode number are different in the first column for `pipe` and `SUB/hard_linked_pipe` and their respective link count was `2` in `SRC` but is now `1` in `DST`. If two processes in different namespaces or chrooted environment, exchange data by mean of such hardlinked pipe, after restoration, if you are not aware of this failure, it will thus be difficult to identify why the two processes are just locked out, one waiting for data that will never come from the pipe, the other stuck for the pipe to be read.

Let's continue by checking the file's metadata:

`root@terre:/mnt/localdisk/Benchmark_tools# getfacl SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- mask::rwx other::r-- # file: DST/plain_zeroed # owner: root # group: root user::rw- <e>user:nobody:rwx group::r-- mask::rwx other::r-- root@terre:/mnt/localdisk/Benchmark_tools# getfattr -d SRC/plain_zeroed DST/plain_zeroed # file: SRC/plain_zeroed <e>user.hello="hello world!!!" # file: DST/plain_zeroed <e>user.hello="hello world!!!" root@terre:/mnt/localdisk/Benchmark_tools# lsattr SRC/plain_zeroed DST/plain_zeroed s---i-d-------e---- SRC/plain_zeroed --------------e---- DST/plain_zeroed root@terre:/mnt/localdisk/Benchmark_tools#`

Note that without `--xattrs` at creation time the timestamp accuracy of tar is 1 second: `root@terre:/mnt/localdisk/Benchmark_tools# stat SRC/random DST/random File: SRC/random Size: 1048576 Blocks: 2048 IO Block: 4096 regular file Device: 802h/2050d Inode: 414841 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) Access: 2020-10-27 14:03:46.064046436 +0100 <e>Modify: 2020-10-27 14:03:42.016050420 +0100 Change: 2020-10-27 14:03:44.048048418 +0100 Birth: - File: DST/random Size: 1048576 Blocks: 2048 IO Block: 4096 regular file Device: 802h/2050d Inode: 414890 Links: 1 Access: (0644/-rw-r--r--) Uid: (65534/ nobody) Gid: ( 0/ root) Access: 2020-10-27 19:08:14.932424226 +0100 <e>Modify: 2020-10-27 14:03:42.000000000 +0100 Change: 2020-10-27 19:08:14.932424226 +0100 Birth: - root@terre:/mnt/localdisk/Benchmark_tools#`

From the above output we see that:

- permission are restored,
- user and group ownership are restored too,
- mtime is restored but it needs `--xattrs` to take into account today's system common time accuracy of one nanosecond
- ACL are restored,
- Extended Attributes are restored

But

- filesystem attributes are not restored,
- atime is not restored,
- ctime is not restored

For the last date, birthtime again we will perform the test under FreeBSD:

`root@FreeBSD:~denis # which mystat mystat: aliased to stat -f "%N%nAccess: %Sa%nModify: %Sm%nChange: %Sc%nBirth: %SB%n" !* root@FreeBSD:~denis # mystat SRC/random SRC/random Access: Oct 27 19:40:13 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 22 15:34:09 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:~denis # cd SRC root@FreeBSD:~denis/SRC # gtar -cf ../backup.tar random root@FreeBSD:~denis/SRC # cd .. root@FreeBSD:~denis # mkdir DST root@FreeBSD:~denis # cd DST root@FreeBSD:~denis/DST # tar -xf ../backup.tar root@FreeBSD:~denis/DST # cd .. root@FreeBSD:~denis # mystat DST/random DST/random Access: Oct 28 15:43:30 2020 Modify: Oct 22 15:34:07 2020 Change: Oct 28 15:43:30 2020 <e>Birth: Oct 22 15:34:07 2020 root@FreeBSD:~denis #`

gtar saved and restored the birthtime

### Feature set

#### Historization

To evaluate this feature, in a first time we will create two files A.txt and B.txt and make a first backup. Then we remove A.txt and add C.txt then make a second backup. We should be able to restore the data in both states (A+B and B+C). To simplify the operation we use the historization_feature script described at the end of this document.

##### Dar

`root@terre:/mnt/memdisk# rm -rf SRC root@terre:/mnt/memdisk# ./historization_feature SRC phase1 root@terre:/mnt/memdisk# dar -c full -g SRC -q root@terre:/mnt/memdisk# ./historization_feature SRC phase2 root@terre:/mnt/memdisk# dar -c diff -A full -g SRC -q root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# dar -x full -R DST -q root@terre:/mnt/memdisk# ls -lR DST DST: total 0 drwxr-xr-x 2 root root 80 Nov 6 18:37 SRC DST/SRC: total 8 -rw-r--r-- 1 root root 13 Nov 6 18:37 <e>A.txt -rw-r--r-- 1 root root 24 Nov 6 18:37 <e>B.txt root@terre:/mnt/memdisk# dar -x diff -R DST -w -q root@terre:/mnt/memdisk# ls -lR DST DST: total 0 drwxr-xr-x 2 root root 80 Nov 6 18:38 SRC DST/SRC: total 8 -rw-r--r-- 1 root root 24 Nov 6 18:37 <e>B.txt -rw-r--r-- 1 root root 21 Nov 6 18:38 <e>C.txt root@terre:/mnt/memdisk#`

Historization is present, we can get back from backup both saved states

In complement dar proposes a manager dar_manager to easily locate file's status between the archives the database has been feeded with, as well as the file's data present in each archive:

`root@terre:/mnt/memdisk# dar_manager -C base.dmd root@terre:/mnt/memdisk# dar_manager -B base.dmd -A full root@terre:/mnt/memdisk# dar_manager -B base.dmd -A diff root@terre:/mnt/memdisk# dar_manager -B base.dmd -f SRC/A.txt 1 Fri Nov 6 18:37:51 2020 saved absent 2 Fri Nov 6 18:38:04 2020 removed absent root@terre:/mnt/memdisk# dar_manager -B base.dmd -f SRC/B.txt 1 Fri Nov 6 18:37:51 2020 saved absent 2 Fri Nov 6 18:37:51 2020 present absent root@terre:/mnt/memdisk# dar_manager -B base.dmd -f SRC/C.txt 2 Fri Nov 6 18:38:04 2020 saved absent root@terre:/mnt/memdisk# dar_manager -B base.dmd -l dar path : dar options : database version: 5 compression used: gzip archive # | path | basename ------------+--------------+--------------- 1 . full 2 . diff root@terre:/mnt/memdisk# dar_manager -B base.dmd -u 1 [ Saved ][ ] SRC/B.txt [ Saved ][ ] SRC/A.txt root@terre:/mnt/memdisk# dar_manager -B base.dmd -u 2 [ Saved ][ ] SRC [ Saved ][ ] SRC/C.txt root@terre:/mnt/memdisk#`

dar_manager can even take for you the actions to invoke dar as many time as necessary get the file's status of a given date for a given set of subset of the saved files:

`root@terre:/mnt/memdisk# dar_manager -v -B base.dmd -e "-R DST -w" -r SRC Decompressing and loading database to memory... Looking in archives for requested files, classifying files archive by archive... Checking chronological ordering of files between the archives... File recorded as removed at this date in database: SRC/A.txt CALLING DAR: restoring 1 files from archive ./full using anonymous pipe to transmit configuration to the dar process Arguments sent through anonymous pipe are: dar -x ./full -R DST -w -g SRC/B.txt -------------------------------------------- 2 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 1 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 3 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- CALLING DAR: restoring 2 files from archive ./diff using anonymous pipe to transmit configuration to the dar process Arguments sent through anonymous pipe are: dar -x ./diff -R DST -w -g SRC -g SRC/C.txt Error while restoring /mnt/memdisk/DST/SRC/A.txt : Cannot remove non-existent file from filesystem: /mnt/memdisk/DST/SRC/A.txt -------------------------------------------- 2 inode(s) restored including 0 hard link(s) 1 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 1 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 4 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- Final memory cleanup... All files asked could not be restored DAR sub-process has terminated with exit code 5 Continue anyway ? [return = YES | Esc = NO] Continuing... root@terre:/mnt/memdisk# ls -lR DST DST: total 0 drwxr-xr-x 2 root root 80 Nov 6 18:38 SRC DST/SRC: total 8 -rw-r--r-- 1 root root 24 Nov 6 18:37 <e>B.txt -rw-r--r-- 1 root root 21 Nov 6 18:38 <e>C.txt root@terre:/mnt/memdisk#`

##### Rsync

`root@terre:/mnt/memdisk# ./historization_feature SRC phase1 root@terre:/mnt/memdisk# rsync -arvHAX SRC DST sending incremental file list created directory DST SRC/ SRC/A.txt SRC/B.txt sent 229 bytes received 84 bytes 626.00 bytes/sec total size is 37 speedup is 0.12 root@terre:/mnt/memdisk# ./historization_feature SRC phase2 root@terre:/mnt/memdisk# rsync -arvHAX SRC DST sending incremental file list SRC/ SRC/C.txt sent 172 bytes received 39 bytes 422.00 bytes/sec total size is 45 speedup is 0.21 root@terre:/mnt/memdisk# ls -l total 4 drwxr-xr-x 3 root root 60 Nov 6 17:06 DST drwxr-xr-x 2 root root 80 Nov 6 17:06 SRC -rwxr--r-- 1 root root 589 Nov 6 16:32 historization_feature root@terre:/mnt/memdisk# ls -l DST total 0 drwxr-xr-x 2 root root 100 Nov 6 17:06 SRC root@terre:/mnt/memdisk# ls -l DST/SRC total 12 <e>-rw-r--r-- 1 root root 13 Nov 6 17:05 A.txt <e>-rw-r--r-- 1 root root 24 Nov 6 17:05 B.txt <e>-rw-r--r-- 1 root root 21 Nov 6 17:06 C.txt root@terre:/mnt/memdisk# rsync -arvHAX --delete SRC DST sending incremental file list deleting SRC/A.txt sent 101 bytes received 26 bytes 254.00 bytes/sec total size is 45 speedup is 0.35 root@terre:/mnt/memdisk# ls -l DST/SRC total 8 <e>-rw-r--r-- 1 root root 24 Nov 6 17:05 B.txt <e>-rw-r--r-- 1 root root 21 Nov 6 17:06 C.txt root@terre:/mnt/memdisk#`

the "backup" contains all three files, A.txt, B.txt and C.txt while the first and the later never existed at the same time. Such backup does not allow to have neither the state of the phase1 nor the state of the phase2.

We added the `--delete` option and as result we got to be the phase2 state only. But then we cannot restore to the phase1 state as the file A.txt has been deleted from the backup.

To have both states with rsync, we should call rsync to a different destination directory at each new backup time, which would consume a lot of space and would also defeat one of the main features of rsync which is its ability to synchronize two directories exchanging only the minimal information that was modified.

##### Tar

`root@terre:/mnt/memdisk# rmdir SRC root@terre:/mnt/memdisk# ./historization_feature SRC phase1 root@terre:/mnt/memdisk# tar --listed-incremental=snapshot.file -cf full.tar SRC root@terre:/mnt/memdisk# ./historization_feature SRC phase2 root@terre:/mnt/memdisk# tar --listed-incremental=snapshot.file -cf diff.tar SRC root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# cd DST root@terre:/mnt/memdisk/DST# tar --listed-incremental=snapshot.file -xf ../full.tar root@terre:/mnt/memdisk/DST# ls -l SRC total 8 <e>-rw-r--r-- 1 root root 13 Nov 6 18:20 A.txt <e>-rw-r--r-- 1 root root 24 Nov 6 18:20 B.txt root@terre:/mnt/memdisk/DST# tar --listed-incremental=snapshot.file -xf ../diff.tar root@terre:/mnt/memdisk/DST# ls -l SRC total 8 <e>-rw-r--r-- 1 root root 24 Nov 6 18:20 B.txt <e>-rw-r--r-- 1 root root 21 Nov 6 18:21 C.txt root@terre:/mnt/memdisk/DST#`

We could restore from backup both the phase1 and phase2 status, historization is available with tar.

#### Data filtering by directory

##### Dar

We want to save `/lib` except the content of `/lib/modules`:

`root@terre:/mnt/memdisk# dar -c backup -R /lib -P modules -vs -q <e>Skipping file: /lib/modules root@terre:/mnt/memdisk#`

What if we want to exclude all of to exclude `/lib/module` except `/lib/module/4.19.0-12-amd64`?

`root@terre:/mnt/memdisk# rm backup.1.dar rm: remove regular file 'backup.1.dar'? y root@terre:/mnt/memdisk# dar -c backup -R /lib -am -P modules -g modules/4.19.0-12-amd64 -vs -q <e>Skipping file: /lib/modules/4.19.0-11-amd64 <e>Skipping file: /lib/modules/4.19.0-10-amd64 root@terre:/mnt/memdisk#`

OK, we can mix included directories and excluded directories

##### Rsync

`root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# rsync -arHAXS --exclude /lib/modules /lib DST root@terre:/mnt/memdisk# ls -ld DST/lib/m* drwxr-xr-x 2 root root 80 Jun 11 23:33 DST/lib/modprobe.d root@terre:/mnt/memdisk# root@terre:/mnt/memdisk# ls -l DST/lib/modules ls: cannot access <e>'DST/lib/modules': No such file or directory root@terre:/mnt/memdisk#`

We could exclude `/lib/modules` as expected. As previously, let's exclude it except `/lib/modules/4.19.0-12-amd64`:

`root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# rsync -arHAXS -f "+ /lib/modules" -f "- /lib/modules/4.19.0-12-amd64" /lib DST root@terre:/mnt/memdisk# la DST/lib/modules/ total 0 drwxr-xr-x 4 root root 80 Oct 22 10:33 . drwxr-xr-x 19 root root 420 Oct 22 11:20 .. <e>drwxr-xr-x 3 root root 280 Aug 8 12:58 4.19.0-10-amd64 <e>drwxr-xr-x 3 root root 280 Oct 12 11:25 4.19.0-11-amd64 root@terre:/mnt/memdisk#`

OK, we can mix included directories and excluded directories

##### Tar

Let's save /lib and excluding /lib/module again:

`root@terre:/mnt/memdisk# tar --exclude /lib/modules -cf backup.tar /lib tar: Removing leading `/' from member names root@terre:/mnt/memdisk# tar -tf backup.tar | grep modules root@terre:/mnt/memdisk#`

Now let's exclude `/lib/modules` except `/lib/modules/4.19.0-12-amd64`:

`root@terre:/mnt/memdisk# tar -cf backup.tar /lib/modules/4.19.0-12-amd64/ --exclude /lib/modules /lib tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets root@terre:/mnt/memdisk# tar -tf backup.tar | wc -l 6017 root@terre:/mnt/memdisk# tar -tf backup.tar | grep -v "lib/modules/4.19.0-12-amd64" | wc -l 1626 root@terre:/mnt/memdisk# tar -tf backup.tar | grep "lib/modules/4.19.0-12-amd64" | wc -l 4391 root@terre:/mnt/memdisk# tar -tf backup.tar | grep "lib/modules" | wc -l 4391 root@terre:/mnt/memdisk#`

The backup contains a total of 6017 entries, 1626 are out of the `lib/modules/4.19.0-12-amd64` directory, the rest is all in that previous directory, nothing else is found in `lib/modules` while there was `lib/modules/4.19.0-11-amd64` and `lib/modules/4.19.0-10-amd64` subdirectory. We can thus mix included and included directories.

#### Data filtering by filename

##### Dar

`root@terre:/mnt/memdisk# dar -c backup -R /lib -X "*.ko" -------------------------------------------- 4122 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) <e>10677 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 14799 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 3945 inode(s) -------------------------------------------- root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# dar -x backup -R DST --fsa-scope none -------------------------------------------- 4122 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 4122 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- root@terre:/mnt/memdisk# find DST -name "*.ko" -ls root@terre:/mnt/memdisk#`

we would exclude all file having the `ko` extension, what if we do not want to exclude those that start with `ext`?

`root@terre:/mnt/memdisk# dar -c backup -R /lib -am -X "*.ko" -I "ext*" -------------------------------------------- 4128 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 10671 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 14799 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 3951 inode(s) -------------------------------------------- root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# dar -x backup -R DST --fsa-scope none -q root@terre:/mnt/memdisk# find DST -name "*.ko" -print DST/modules/4.19.0-10-amd64/kernel/fs/ext4/ext4.ko DST/modules/4.19.0-10-amd64/kernel/drivers/extcon/extcon-core.ko DST/modules/4.19.0-11-amd64/kernel/fs/ext4/ext4.ko DST/modules/4.19.0-11-amd64/kernel/drivers/extcon/extcon-core.ko DST/modules/4.19.0-12-amd64/kernel/fs/ext4/ext4.ko DST/modules/4.19.0-12-amd64/kernel/drivers/extcon/extcon-core.ko root@terre:/mnt/memdisk#`

OK, we got what we wanted

##### Rsync

`root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# rsync -arHAXS -f "- *.ko" /lib DST root@terre:/mnt/memdisk# find DST -name "*.ko" -print root@terre:/mnt/memdisk# ls DST lib root@terre:/mnt/memdisk#`

Same as previously, we don't want to exclude `ko` files starting by `ext`:

`root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# rsync -arHAXS -f "+ ext*" -f "- *.ko" /lib DST root@terre:/mnt/memdisk# find DST -name "*.ko" -print DST/lib/modules/4.19.0-12-amd64/kernel/fs/ext4/ext4.ko DST/lib/modules/4.19.0-12-amd64/kernel/drivers/extcon/extcon-core.ko DST/lib/modules/4.19.0-11-amd64/kernel/fs/ext4/ext4.ko DST/lib/modules/4.19.0-11-amd64/kernel/drivers/extcon/extcon-core.ko DST/lib/modules/4.19.0-10-amd64/kernel/fs/ext4/ext4.ko DST/lib/modules/4.19.0-10-amd64/kernel/drivers/extcon/extcon-core.ko root@terre:/mnt/memdisk#`

OK, we got what we wanted

##### Tar

Same as previously, let's filter out kernel object files

`root@terre:/mnt/memdisk# tar -cf backup.tar --exclude "*.ko" /lib tar: Removing leading `/' from member names root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# mkdir DST root@terre:/mnt/memdisk# cd DST root@terre:/mnt/memdisk/DST# tar -xf ../backup.tar root@terre:/mnt/memdisk/DST# find . -name "*.ko" -print root@terre:/mnt/memdisk/DST#`

Now, we want to keep only those kernel object files starting with `ext`

`root@terre:/mnt/memdisk# tar -cf backup.tar "ext*" --exclude "*.ko" /lib tar: ext*: Cannot stat: No such file or directory tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets tar: Exiting with failure status due to previous errors root@terre:/mnt/memdisk#`

Well, argument passed out of option do not seem expanded by tar thus using mask is not possible to include some pattern. It seems the only option is to use file listing, thing we will evaluate below.

#### Data filtering by filesystem

We will use a tmpfs filesystem mounted twice thanks to mount's --bind option. The objective is first to save every thing except a few given filesystems, or only one or save inside a few given filesystems. Here is the preparation phase:

`root@terre:/mnt/memdisk# mkdir SRC root@terre:/mnt/memdisk# mkdir SRC/D1 SRC/D2 SRC/D3 root@terre:/mnt/memdisk# mount -t tmpfs tmpfs SRC/D1 root@terre:/mnt/memdisk# mount --bind SRC/D1 SRC/D2 root@terre:/mnt/memdisk# mount -t tmpfs tmpfs SRC/D3 root@terre:/mnt/memdisk# ls SRC/D1 SRC/D2 SRC/D1: SRC/D2: root@terre:/mnt/memdisk# echo "Hello World" > SRC/D1/file.txt root@terre:/mnt/memdisk# ls SRC/D1 SRC/D2 SRC/D1: file.txt SRC/D2: file.txt root@terre:/mnt/memdisk# echo "give me your data, I'll tell your needs and what to buy" > SRC/gafam.com root@terre:/mnt/memdisk# echo "sight" > SRC/D3/democracy.org root@terre:/mnt/memdisk#`

##### Dar

`root@terre:/mnt/memdisk# dar -c backup -R SRC -MX:/mnt/memdisk/SRC/D1 -vs -vt -q Adding folder to archive: /mnt/memdisk/SRC/D3 Adding file to archive: /mnt/memdisk/SRC/D3/democracy.org Adding file to archive: /mnt/memdisk/SRC/gafam.com Skipping file: /mnt/memdisk/SRC/D2 Skipping file: /mnt/memdisk/SRC/D1 root@terre:/mnt/memdisk#`

We could exclude a filesystem, and its second appearance in D2 was also excluded, whithout having to mention it. Let's include only `D1` now:

`root@terre:/mnt/memdisk# rm -f backup.1.dar root@terre:/mnt/memdisk# dar -c backup -R SRC -MI:/mnt/memdisk/SRC/D1 -vs -vt -q Skipping file: /mnt/memdisk/SRC/D3 Adding file to archive: /mnt/memdisk/SRC/gafam.com Adding folder to archive: /mnt/memdisk/SRC/D2 Adding file to archive: /mnt/memdisk/SRC/D2/file.txt Adding folder to archive: /mnt/memdisk/SRC/D1 Adding file to archive: /mnt/memdisk/SRC/D1/file.txt root@terre:/mnt/memdisk#`

OK, we got what we wanted

##### Rsync

`root@terre:/mnt/memdisk# rsync -arvHAXS --one-file-system SRC DST sending incremental file list created directory DST SRC/ SRC/gafam.com SRC/D1/ SRC/D2/ SRC/D3/ sent 283 bytes received 77 bytes 720.00 bytes/sec total size is 56 speedup is 0.16 root@terre:/mnt/memdisk#`

rsync has ony one option about filesystems, it sticks recursion to the filesystem of the source directory, we cannot exclude specifically some filesystems, they are all excluded, and we cannot include specifically some filesystems, none is excluded (default behavior without this option)

##### Tar

`root@terre:/mnt/memdisk# tar -cvf backup.tar --one-file-system SRC SRC/ SRC/D3/ tar: SRC/D3/: file is on a different filesystem; not dumped SRC/gafam.com SRC/D2/ tar: SRC/D2/: file is on a different filesystem; not dumped SRC/D1/ tar: SRC/D1/: file is on a different filesystem; not dumped root@terre:/mnt/memdisk#`

tar does not behaves better than rsync on that topic

#### Data filtering by tag

by tag we mean any mark the user can add to a file that will drive its fate when backup will be done. The most common is the dump flag, but it is not always available, using some other mechanisms (Extended Attributes,...) can be an interesting alternative.

##### Dar

`root@terre:/var/tmp# mkdir SRC root@terre:/var/tmp# echo "Hello" > file1.txt root@terre:/var/tmp# echo "World" > file2.txt root@terre:/var/tmp# chattr +d file1.txt root@terre:/var/tmp# setfattr -n <e>user.no_dump file2.txt root@terre:/var/tmp# mv file1.txt file2.txt SRC root@terre:/var/tmp# dar -c backup -w -R SRC --nodump -vt -q <e>Adding file to archive: /var/tmp/SRC/file2.txt Saving Extended Attributes for /var/tmp/SRC/file2.txt Saving Filesystem Specific Attributes for /var/tmp/SRC/file2.txt root@terre:/var/tmp# dar -c backup -w -R SRC <e>--exclude-by-ea=user.no_dump -vt -q Adding file to archive: /var/tmp/SRC/file1.txt Saving Filesystem Specific Attributes for /var/tmp/SRC/file1.txt root@terre:/var/tmp#`

We have two mechanisms, one based on the dump flag and an arbitrary extended attribute. However dar only supports exclusion of file, not inclusion for backup based on a tag.

##### Rsync

rsync does not seem to be able to filter based on an arbitrary mark

##### Tar

rsync does not seem to be able to filter based on an arbitrary mark

#### Data filtering by files listing

We build a file listing and expect to either have only those file saved or excluded from the performed backup. Here is the listing preparation:

`root@terre:/mnt/memdisk# find /lib -name "*.ko" -o -print > include.txt root@terre:/mnt/memdisk# wc -l include.txt 4123 include.txt root@terre:/mnt/memdisk# find /lib -name "*.ko" -print > exclude.txt root@terre:/mnt/memdisk# wc -l exclude.txt 10677 exclude.txt root@terre:/mnt/memdisk#`

##### Dar

`root@terre:/mnt/memdisk# dar -c backup -R /lib -[ include.txt -------------------------------------------- <e>4122 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) <e>10677 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 14799 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 3945 inode(s) -------------------------------------------- root@terre:/mnt/memdisk#`

file inclusion is available, let's see file exclusion:

`root@terre:/mnt/memdisk# dar -c backup -R /lib -] exclude.txt -------------------------------------------- <e>4122 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) <e>10677 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 14799 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 3945 inode(s) -------------------------------------------- root@terre:/mnt/memdisk#`

##### Rsync

`root@terre:/mnt/memdisk# rsync -aHAXS --files-from=include.txt / DST root@terre:/mnt/memdisk# find DST -print | wc -l 4124 root@terre:/mnt/memdisk# find DST -name "*.ko" -print root@terre:/mnt/memdisk#`

file inclusion is available. However if we can exclude a list of pattern defined in a file, we cannot exclude a list of files. We should prepend each entry by "- " seen the filtering syntax of rsync:

`root@terre:/mnt/memdisk# sed -r 's/^/- /' exclude.txt > rsync-exclude.txt root@terre:/mnt/memdisk# rm -rf DST root@terre:/mnt/memdisk# rsync -aHAXS --exclude-from=rsync-exclude.txt /lib DST root@terre:/mnt/memdisk# find DST -print | wc -l 4124 root@terre:/mnt/memdisk# find DST -name "*.ko" -print | wc -l 0 root@terre:/mnt/memdisk#`

So we are good with this sed additional listing adaptation.

##### Tar

`root@terre:/mnt/memdisk# tar -cvf backup.tar --files-from=include.txt | wc -l tar: Removing leading `/' from member names 98017 root@terre:/mnt/memdisk# tar -tf backup.tar | grep .ko | wc -l 73392 root@terre:/mnt/memdisk# grep .ko include.txt | wc -l 3 root@terre:/mnt/memdisk# grep .ko include.txt /lib/modules/4.19.0-12-amd64/kernel/sound/pci/korg1212 /lib/modules/4.19.0-11-amd64/kernel/sound/pci/korg1212 /lib/modules/4.19.0-10-amd64/kernel/sound/pci/korg1212 root@terre:/mnt/memdisk#`

the include.txt file does not contain any file with the ko extension, however tar saved all of them. Reading back the man page concerning this --files-from option The names read are handled the same way as command line arguments explains that in the listing where all `"*.ko"` files have been removed, remain their parent directory, which implies saving all its content. In consequence we must not list directories only their content (which will restrict us saving empty directories as such). Let's modify the include.txt file that way:

`find /lib -type d -o -name "*.ko" -o -print > tar-include.txt root@terre:/mnt/memdisk# tar -cvf backup.tar --files-from=tar-include.txt | wc -l tar: Removing leading `/' from member names 1532 root@terre:/mnt/memdisk# tar -tvf backup.tar | grep "*.ko" root@terre:/mnt/memdisk# wc -l tar-include.txt 1532 tar-include.txt root@terre:/mnt/memdisk# wc -l include.txt 4123 include.txt`

the difference between the 1532 entries saved by tar and the 4123 saved by rsync or dar comes from the many empty directories that cannot be saved as such by tar using this method.

`root@terre:/mnt/memdisk# tar -cvf backup.tar --exclude-from=exclude.txt /lib | wc -l tar: Removing leading `/' from member names 4123 root@terre:/mnt/memdisk# wc -l exclude.txt 10677 exclude.txt root@terre:/mnt/memdisk# tar -tf backup.tar | egrep "\.ko$" root@terre:/mnt/memdisk#`

The file listing exclusion works as expected

#### Slicing

For this test we will backup the content of /usr/bin of the running system. We select a slice size smaller than the biggest file under backup. The use case for slicing implies compression (remote storage, cloud storage, limited removable media storage...).

`root@terre:/mnt/memdisk# ls -lh --sort=size /usr/bin | tac | tail -rwxr-xr-x 1 root root 8.0M Dec 18 2018 luajittex -rwxr-xr-x 1 root root 8.1M Dec 18 2018 luatex53 -rwxr-xr-x 1 root root 8.1M Dec 18 2018 luatex -rwxr-xr-x 1 root root 8.2M May 27 2019 wireshark -rwxr-xr-x 1 root root 12M Dec 21 2018 kstars -rwxr-xr-x 1 root root 15M Mar 12 2018 doxygen -rwxr-xr-x 1 root root 16M Jan 4 2019 stellarium -rwxr-xr-x 1 root root 19M Oct 12 19:46 mysql_embedded -rwxr-xr-x 1 root root <e>39M Sep 5 2019 emacs-gtk total 430M root@terre:/mnt/memdisk#`

##### Dar

`terre:/mnt/memdisk# dar -c backup -R /usr/bin -z6 <e>-s 20M -q terre:/mnt/memdisk# ls -lh backup.* -rw-r--r-- 1 root root <e>20M Nov 13 11:30 backup.1.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.2.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.3.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.4.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.5.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.6.dar -rw-r--r-- 1 root root 20M Nov 13 11:30 backup.7.dar -rw-r--r-- 1 root root <e>7.7M Nov 13 11:30 backup.8.dar terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# dar -x backup -R DST -g emacs-gtk <e>-E "echo openning slice %p/%b.%N.%e" <e>openning slice /mnt/memdisk/backup.8.dar <e>openning slice /mnt/memdisk/backup.4.dar <e>openning slice /mnt/memdisk/backup.5.dar Restoration of FSA for /mnt/memdisk/DST/emacs-gtk aborted: Failed reading existing extX family FSA: Inappropriate ioctl for device Restoration of linux immutable FSA for /mnt/memdisk/DST/emacs-gtk aborted: Failed reading existing extX family FSA: Inappropriate ioctl for device -------------------------------------------- 1 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 2591 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 2592 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# diff DST/emacs-gtk /usr/bin/emacs-gtk memdiskerre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

We can also specify a different size for the first slice when using dar, this was used in the past to fulfill a disk partially filled by a previous incremental backup when saving onto CD-RW and DVD-RW, but that may still make sense when using USB keys or any other removable media.

`root@terre:/mnt/memdisk# dar -c backup -R /usr/bin -s 20M <e>-S 1M -q --min-digit 3 root@terre:/mnt/memdisk# ls -lh total 361M -rw-r--r-- 1 root root <e>1.0M Nov 6 18:57 backup.001.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.002.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.003.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.004.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.005.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.006.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.007.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.008.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.009.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.010.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.011.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.012.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.013.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.014.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.015.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.016.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.017.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.018.dar -rw-r--r-- 1 root root 20M Nov 6 18:57 backup.019.dar root@terre:/mnt/memdisk# dar -c backup -R /usr/bin -s 20M <e>-S 200M -q --min-digit 3 root@terre:/mnt/memdisk# ls -lh total 361M -rw-r--r-- 1 root root <e>200M Nov 6 18:58 backup.001.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.002.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.003.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.004.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.005.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.006.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.007.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.008.dar -rw-r--r-- 1 root root 20M Nov 6 18:59 backup.009.dar -rw-r--r-- 1 root root 913K Nov 6 18:59 backup.010.dar root@terre:/mnt/memdisk#`

##### Rsync

rsync cannot split any file in slices, and it does not generate any backup, but it copies files. You cannot thus split data into slices to fit a particular restricted storage space.

##### Tar

`root@terre:/mnt/memdisk# tar -czf backup.tar -M -L 20480 /usr/bin tar: Cannot use multi-volume compressed archives Try 'tar --help' or 'tar --usage' for more information. root@terre:/mnt/memdisk#`

As reported by tar above, if a multi-volume support exists, it is quite restrictive as one cannot use compression at the same time.

`terre:/mnt/memdisk# tar -cf backup -M -L 20480 /usr/bin tar: Removing leading `/' from member names Prepare volume #2 for 'backup' and hit return: tar: Removing leading `/' from hard link targets Prepare volume #3 for 'backup' and hit return: Prepare volume #4 for 'backup' and hit return: Prepare volume #5 for 'backup' and hit return: Prepare volume #6 for 'backup' and hit return: Prepare volume #7 for 'backup' and hit return: Prepare volume #8 for 'backup' and hit return: Prepare volume #9 for 'backup' and hit return: Prepare volume #10 for 'backup' and hit return: Prepare volume #11 for 'backup' and hit return: Prepare volume #12 for 'backup' and hit return: Prepare volume #13 for 'backup' and hit return: Prepare volume #14 for 'backup' and hit return: Prepare volume #15 for 'backup' and hit return: Prepare volume #16 for 'backup' and hit return: Prepare volume #17 for 'backup' and hit return: Prepare volume #18 for 'backup' and hit return: Prepare volume #19 for 'backup' and hit return: Prepare volume #20 for 'backup' and hit return: Prepare volume #21 for 'backup' and hit return: terre:/mnt/memdisk# terre:/mnt/memdisk# ls -l backup* -rw-r--r-- 1 root root 19527680 Nov 13 11:40 backup terre:/mnt/memdisk#`

But even without compression, tar is still restrictive: it does not produce different files, you have each new volume around and `hit return` at each time.
Note also that without compression, the space required passes from 8 volumes with dar to 21 volumes with tar.

The multi-volume support for tar seems well defined for local tape removable devices, but will cost more than twice more tape than what you can do with dar even if tape media is your only target. Here is an example with dar on how to write to mutli-volume and compressed backup to tape and pause between each volume as tar does:

`terre:/mnt/memdisk# <e>dar -c backup -R /usr/bin -z6 -s 20M -E "echo writing volume %N to tape" <e>-E "cat < %p/%b.%N.%e > /dev/mt" -p class=green>writing volume 1 to tape Finished writing to file 1, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 2 to tape Finished writing to file 2, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 3 to tape Finished writing to file 3, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 4 to tape Finished writing to file 4, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 5 to tape Finished writing to file 5, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 6 to tape Finished writing to file 6, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 7 to tape Finished writing to file 7, ready to continue ? [return = YES | Esc = NO] Continuing... writing volume 8 to tape -------------------------------------------- 2592 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2592 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2152 inode(s) -------------------------------------------- terre:/mnt/memdisk#`

#### Symmetric encryption

Encryption has for target relatively long term lifetime, having compression at the same time to increase security as it increases data "randomness" of the data to cipher. So we will use both in our tests (gzip with a compresion level of 6).

A point to pay attention to concerns the way the password/passphrase can be provided. Putting this to the command-line could let other users on this same system read it. Having interactive prompt is better as well as having the password set in a read access restricted file, which in addition allows automation.

##### Dar

`root@terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -K aes256: -q -z6 Archive backup <e>requires a password: Please <e>confirm your password: root@terre:/mnt/memdisk# dar -l backup -q Archive backup <e>requires a password: Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted Archive version format : 11 Compression algorithm used : <e>gzip Compression block size used : 0 Symmetric key encryption used : <e>AES 256 Asymmetric key encryption used : none Archive is signed : no Sequential reading marks : present User comment : N/A KDF iteration count : <e>10000 KDF hash algorithm : <e>argon2 <e>Salt size : 32 bytes Catalogue size in archive : 101907 bytes Archive is composed of 1 file(s) File size: 155070897 bytes The global data compression ratio is: 64% CATALOGUE CONTENTS : total number of inode : 2589 fully saved : 2589 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 2 - plain files : 2152 - symbolic links : 435 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 5 - number of reference to hard linked inodes: 10 destroyed entries information 0 file(s) have been record as destroyed since backup of reference root@terre:/mnt/memdisk# touch pass.dcf root@terre:/mnt/memdisk# chmod go-rwx pass.dcf root@terre:/mnt/memdisk# cat >> pass.dcf -K "aes256:hello world!" root@terre:/mnt/memdisk# ls -l pass.dcf -rw------- 1 root root 25 Nov 9 11:37 pass.dcf root@terre:/mnt/memdisk# rm backup.1.dar rm: remove regular file 'backup.1.dar'? y root@terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -B pass.dcf -q -z6 root@terre:/mnt/memdisk# dar -l backup -q -B pass.dcf Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted Archive version format : 11 Compression algorithm used : gzip Compression block size used : 0 Symmetric key encryption used : AES 256 Asymmetric key encryption used : none Archive is signed : no Sequential reading marks : present User comment : N/A KDF iteration count : 10000 KDF hash algorithm : argon2 Salt size : 32 bytes Catalogue size in archive : 102310 bytes Archive is composed of 1 file(s) File size: 155132433 bytes The global data compression ratio is: 64% CATALOGUE CONTENTS : total number of inode : 2589 fully saved : 2589 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 2 - plain files : 2152 - symbolic links : 435 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 5 - number of reference to hard linked inodes: 10 destroyed entries information 0 file(s) have been record as destroyed since backup of reference root@terre:/mnt/memdisk#`

We can provide password either on command-line (not recommended), prompted by dar once launched or from a protected configuration file. In the following we add slicing to encryption to see whether or not dar deciphers the whole backup to recover a single file:

`root@terre:/mnt/localdisk# rm -rf backup.* root@terre:/mnt/localdisk# dar -c backup -R / -g usr/bin -K aes256: -s 1M -q -z6 Archive backup requires a password: Please confirm your password: root@terre:/mnt/localdisk# ls -l backup.* | wc -l <e>148 root@terre:/mnt/localdisk# dar -x backup -g usr/bin/emacs-gtk -E "echo openning slice %b.%N.%e" -q openning slice backup.148.dar Archive backup requires a password: Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted <e>openning slice backup.1.dar <e>openning slice backup.80.dar <e>openning slice backup.81.dar <e>openning slice backup.82.dar <e>openning slice backup.83.dar <e>openning slice backup.84.dar root@terre:/mnt/localdisk#`

As seen above, dar does not need to uncipher nor uncompress the whole backup to recover a single file, the use of slicing let us see which slice it accessed to, but the behavior is the same without slicing and can be measure by the execution time (see the performance tests logs).

##### Rsync

rsync cannot cipher data, it can rely on ssh to cipher the data over the network but data is finally always stored in clear text.

##### Tar

There is no native support for ciphering with tar. You can however pipe tar's output to openssl to cipher the generated backup on fly as a whole.

`root@terre:/mnt/memdisk# tar -czf - /usr/bin | openssl enc -e -aes256 -out backup.tar.gz.crypted tar: Removing leading `/' from member names enter aes-256-cbc encryption password: Verifying - enter aes-256-cbc encryption password: <e>*** WARNING : deprecated key derivation used. <e>Using -iter or -pbkdf2 would be better. tar: Removing leading `/' from hard link targets root@terre:/mnt/memdisk# openssl enc -d -aes256 -in backup.tar.gz.crypted | tar -xz enter aes-256-cbc decryption password: <e>*** WARNING : deprecated key derivation used. <e>Using -iter or -pbkdf2 would be better. root@terre:/mnt/memdisk# tar -czf - /usr/bin | openssl enc -e -aes256 -out backup.tar.gz.crypted -pass file:pass.txt tar: Removing leading `/' from member names <e>*** WARNING : deprecated key derivation used. <e>Using -iter or -pbkdf2 would be better. tar: Removing leading `/' from hard link targets root@terre:/mnt/memdisk# openssl enc -d -aes256 -in backup.tar.gz.crypted -pass file:pass.txt | tar -xz <e>*** WARNING : deprecated key derivation used. <e>Using -iter or -pbkdf2 would be better. root@terre:/mnt/memdisk#`

with openssl, tar has both the ability to provide the password/passphrase from an interactive prompt and from a protected file. However you will have to remember which algorithm you used in adition to the passphrase. The ciphering being done as a whole, you will have to decipher the whole backup even to just restore a single file. If the backup is large, this may take a long time and may require to download a lot of stuff from a remote storage.

We see that ciphering with tar is possible at the cost of some complex command-line. But this is error-prone as we see the shown warning that the key derivation function is deprecated and we should switch to another one. Moreover you will have to remember which key derivation function and its parameters in addition to the passphrase you provided and in addition to the ciphering algorithm used.

Note: you can also use openssl with dar as we did for tar but it brings all the drawbacks we saw with tar

#### Asymmetric encryption

The objective is to create a backup ciphered using GnuPG public/private key pair, restore the whole backup and restore a single file from it. We will also use compression (gzip level 6) as it may make sense for the corresponding use cases (data exchange over Internet for example).

##### Dar

`terre:/mnt/memdisk# dar -c backup -K gnupg::root@terre.systeme-solaire.espace -R SRC -z6 -q terre:/mnt/memdisk# dar -l backup -q Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted Archive version format : 11 Compression algorithm used : <e>gzip Compression block size used : 0 Symmetric key encryption used : <e>AES 256 Asymmetric key encryption used : <e>gnupg Archive is signed : no Sequential reading marks : present User comment : N/A Catalogue size in archive : 68669 bytes Archive is composed of 1 file(s) File size: 158261425 bytes The global data compression ratio is: 64% CATALOGUE CONTENTS : total number of inode : 2593 fully saved : 2593 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 1 - plain files : 2157 - symbolic links : 435 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 0 - number of reference to hard linked inodes: 0 destroyed entries information 0 file(s) have been record as destroyed since backup of reference terre:/mnt/memdisk# ls -l backup.1.dar -rw-r--r-- 1 root root 158261425 Nov 9 16:04 backup.1.dar terre:/mnt/memdisk#`

As displayed in the backup header output above the underlying encryption is a symmetric encryption (AES 256 by default), but the AES key is stored ciphered using the private key of the backup recipient which email address is provided (or email adresses, if more than one recipient is expected). This key is randomly chosen by dar and stored ciphered in the archive header. Thus the overall behavior, performance and security of GnuPG withing dar is equivalent to the one of the symmetrical algorithm chosen, with the ability to quickly restore some or all files from an archive, and not waiting/downloading first the whole backup to unciphered it.

Seen above no password or passphrase is asked as the recipient email is ourselves (root@terre.systeme-solaire.espace). Let's cipher for another recipient:

`terre:/mnt/memdisk# dar -c backup -K gnupg::dar.linux@free.fr -R SRC -z6 -q -w terre:/mnt/memdisk# ls -l backup.1.dar -rw-r--r-- 1 root root <e>158230913 Nov 9 16:22 backup.1.dar terre:/mnt/memdisk# dar -l backup -q FATAL error, aborting operation: Unexpected error reported by GPGME: <e>No secret key terre:/mnt/memdisk# dar -c backup -K gnupg::dar.linux@free.fr,root@terre.systeme-solaire.espace -R SRC -z6 -q -w terre:/mnt/memdisk# dar -l backup -q Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted Archive version format : 11 Compression algorithm used : gzip Compression block size used : 0 Symmetric key encryption used : AES 256 Asymmetric key encryption used : gnupg Archive is signed : no Sequential reading marks : present User comment : N/A Catalogue size in archive : 68624 bytes Archive is composed of 1 file(s) File size: 158252223 bytes The global data compression ratio is: 64% CATALOGUE CONTENTS : total number of inode : 2593 fully saved : 2593 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 1 - plain files : 2157 - symbolic links : 435 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 0 - number of reference to hard linked inodes: 0 destroyed entries information 0 file(s) have been record as destroyed since backup of reference terre:/mnt/memdisk#`

Here we saw that ciphering for a recipient different than ourself does not allow us to read the resulting backup, however we can define several recipients and if we add ourself, we can read the backup as well as our primary recipients.

##### Rsync

rsync is not able to perform asymmetric encryption of backed up files.

##### Tar

Tar cannot hold asymmetrical encryption alone, as for symmetrical encryption we must use an external tool that performes the ciphering operation outside the backup.

`terre:/mnt/memdisk# tar -czf - SRC | gpg --encrypt --recipient root@terre.systeme-solaire.espace --output backup.tar.gz.gpg terre:/mnt/memdisk# ls -l backup.tar.gz.gpg -rw-r--r-- 1 root root <e>155337814 Nov 9 16:45 backup.tar.gz.gpg terre:/mnt/memdisk# terre:/mnt/memdisk# gpg --decrypt backup.tar.gz.gpg | tar -xzf - gpg: encrypted with 3072-bit RSA key, ID 97E13D38B007DF30, created 2020-08-08 "root@terre <root@terre.systeme-solaire.espace>" terre:/mnt/memdisk# terre:/mnt/memdisk# tar -czf - SRC | gpg --encrypt --recipient dar.linux@free.fr --output backup.tar.gz.gpg terre:/mnt/memdisk# gpg --decrypt backup.tar.gz.gpg | tar -xzf - gpg: encrypted with 4096-bit RSA key, ID DB0A2141A4D96ECA, created 2012-09-13 "Denis Corbin (http://dar.linux.free.fr/) <dar.linux@free.fr>" gpg: decryption failed: <e>No secret key gzip: stdin: unexpected end of file tar: Child returned status 1 tar: Error is not recoverable: exiting now terre:/mnt/memdisk#terre:/mnt/memdisk# tar -czf - SRC | gpg --encrypt --recipient dar.linux@free.fr \ --recipient root@terre.systeme-solaire.espace --output backup.tar.gz.gpg terre:/mnt/memdisk# gpg --decrypt backup.tar.gz.gpg | tar -xzf - gpg: encrypted with 4096-bit RSA key, ID DB0A2141A4D96ECA, created 2012-09-13 "Denis Corbin (http://dar.linux.free.fr/) <dar.linux@free.fr>" gpg: encrypted with 3072-bit RSA key, ID 97E13D38B007DF30, created 2020-08-08 "root@terre <root@terre.systeme-solaire.espace>" terre:/mnt/memdisk#`

Same as for symmetric encryption, the fact that the whole backup is ciphered at once implies to download back the whole backup even to recover just one file.

#### Protection against plain-text attack

##### Dar

`devuan:/mnt/memdisk# time dar -c backup -K "aes256:hello world!" -at -1 0 -R SRC -q -w 9.782u 3.413s <e>0:06.28 210.0% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# la backup.1.dar -rw-r--r-- 1 root root 1572<e>706497 Nov 9 14:50 backup.1.dar devuan:/mnt/memdisk# time dar -c backup -K "aes256:hello world!" -at -1 0 -R SRC -q -w 9.173u 2.845s <e>0:05.50 218.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# la backup.1.dar -rw-r--r-- 1 root root 1572<e>655217 Nov 9 14:50 backup.1.dar devuan:/mnt/memdisk#`

When ciphering the same data several times (with symmetric or asymmetric encryption), the resulting backup size changes each time. This is due to the garbage (the elastic buffer) dar adds at the beginnning and at the end of the data to cipher. This way, even if a dar backup has well known structure it is not easy to know precisely where they are positionned in the backup file, which makes plain-text attack much more difficult to succeed if even possible in a reasonable time.

##### Rsync

rsync does not provide any way to cipher the backup, it is thus not concerned by protecting against plain-text attack.

##### Tar

`devuan:/mnt/memdisk/SRC# time ../tar.backup ../backup.tar.crypted usr 4.112u 2.343s 0:04.72 136.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC# ls -l ../bac backup.1.dar backup.tar.crypted devuan:/mnt/memdisk/SRC# ls -l ../backup.tar.crypted -rw-r--r-- 1 root root <e>1603594272 Nov 9 14:56 ../backup.tar.crypted devuan:/mnt/memdisk/SRC# time ../tar.backup ../backup.tar.crypted usr 3.952u 2.564s 0:04.79 135.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC# ls -l ../backup.tar.crypted -rw-r--r-- 1 root root <e>1603594272 Nov 9 14:56 ../backup.tar.crypted devuan:/mnt/memdisk/SRC#`

tar by itself does not provide any ciphering mechanism, however you can cipher the tar generated backups with external tool (for example openssl for symmetric encryption or gpg for asymmetric encryption). However none of these mechanism protect against plain-text attack: tar backup have somehow predictable header contents.

#### Key Derivation Function

##### Dar

dar uses `argon2` by default, with 10,000 iterations. It can also use pkcs5 v2 (pbkdf2) with md5, sha1 or sha512 algorithm. The user is able to set the KDF function and iteration count, so we are able to measure the execution time variation added by the iteration count (taking into account that the data to cipher also changes depending on the amount of random garbage dar wraps it with):

`terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>100k:sha1 -w -q 4.904u 0.572s <e>0:05.49 99.6% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>500k:sha1 -w -q 5.805u 0.272s <e>0:06.08 99.8% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>1M:sha1 -w -q 6.852u 0.308s <e>0:07.18 99.5% 0+0k 0+0io 0pf+0w time dar -c backup -R SRC -K aes:hello --kdf-param 10k:argon2 -w -q 5.092u 0.870s 0:03.50 170.2% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>10k:argon2 -w -q 5.232u 0.760s <e>0:03.54 169.2% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>20k:argon2 -w -q 5.778u 0.822s <e>0:04.14 159.1% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>100k:argon2 -w -q 10.613u 0.831s <e>0:09.00 127.1% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# time dar -c backup -R SRC -K aes:hello --kdf-param <e>1M:argon2 -w -q 66.862u 0.666s <e>1:05.14 103.6% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk#`

##### Rsync

rsync does not provide any way to cipher the backup, it is not concerned by KDF.

##### Tar

As of today (year 2020) openssl only supports PBKDF2: no support for argon2 is available. Argon2 was the winner of the Password Hashing Competition in July 2015. PBKDF2 has been published by the IETF in September 2000 with the RCF 2898

#### File change detection

In order stress each backup software on that aspect, we will use an ugly script always_change that loops forever permanently invoking `touch` on a given file. For the test, we create a source tree to backup, containing a file of 1 MiB on which we will apply this script:

`terre:/mnt/memdisk# mkdir SRC terre:/mnt/memdisk# dd if=/dev/zero of=SRC/hello_world bs=10240 count=1024 1024+0 records in 1024+0 records out 10485760 bytes (10 MB, 10 MiB) copied, 0.0107294 s, 977 MB/s terre:/mnt/memdisk# ./always_change SRC/hello_world & [1] 7433 terre:/mnt/memdisk# stat SRC/hello_world File: SRC/hello_world Size: 10485760 Blocks: 20480 IO Block: 4096 regular file Device: 1bh/27d Inode: 375588 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2020-11-10 16:34:13.806106695 +0100 <e>Modify: 2020-11-10 16:34:13.806106695 +0100 Change: 2020-11-10 16:34:13.806106695 +0100 Birth: - terre:/mnt/memdisk# stat SRC/hello_world File: SRC/hello_world Size: 10485760 Blocks: 20480 IO Block: 4096 regular file Device: 1bh/27d Inode: 375588 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2020-11-10 16:34:14.838104981 +0100 <e>Modify: 2020-11-10 16:34:14.838104981 +0100 Change: 2020-11-10 16:34:14.838104981 +0100 Birth: - terre:/mnt/memdisk# jobs [1] + Running ./always_change SRC/hello_world terre:/mnt/memdisk# ls -l SRC total 10240 -rw-r--r-- 1 root root 10485760 Nov 10 16:34 hello_world terre:/mnt/memdisk#`

##### Dar

`terre:/mnt/memdisk# dar -c backup -R SRC -q <e>WARNING! File modified while reading it for backup, but no more retry allowed: /mnt/memdisk/SRC/hello_world terre:/mnt/memdisk# dar -l backup [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [<e>DIRTY][ ] [---][ 99%][X] -rw-r--r-- 0 0 10 Mio Tue Nov 10 16:34:55 2020 hello_world terre:/mnt/memdisk# dar -x backup -R DST <e>File /mnt/memdisk/DST/hello_world has changed during backup and is probably not saved in a valid state ("dirty file"), <e>do you want to consider it for restoration anyway? [return = YES | Esc = NO] Continuing... -------------------------------------------- 1 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 1 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk#`

- dar detects properly the file change and issues a warning during the backup.
- It even retries to save the file several times (3 times by default).
- the resulting backup keeps trace of this context by flagging the file as `DIRTY`
- When restoring the data, a warning shows (default behavior) and the user is requested for confirmation.

##### Rsync

`terre:/mnt/memdisk# rsync -arvHAXqz --delete SRC DST terre:/mnt/memdisk#`

rsync does not shows anything nor behaves differently (no retry, no change notification).

##### Tar

`terre:/mnt/memdisk# tar -cf backup.tar SRC <e>tar: SRC/hello_world: file changed as we read it terre:/mnt/memdisk# tar -tvf backup.tar drwxr-xr-x root/root 0 2020-11-10 16:33 SRC/ -rw-r--r-- root/root 10485760 2020-11-10 16:41 SRC/hello_world terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xf ../backup.tar terre:/mnt/memdisk/DST# ls -l SRC total 10240 -rw-r--r-- 1 root root 10485760 Nov 10 16:41 hello_world terre:/mnt/memdisk/DST#`

- tar detects properly the file change and issues a warning during the backup.
- it does not tries to save the file
- the resulting backup keeps no visible trace of this possible data corruption
- When restoring the data, no warning is issued and the restoration proceed as if the file was saved properly

#### Multi-level backup

For this test we make a full backup of a Linux source tree, then rename the Documentation directory as doc and make a differential backup of the whole. Renaming files is expected to do at worse the same as removing some and adding new ones, whe should not see all data saved again:

##### Dar

`devuan:/mnt/memdisk# du -B1 -s SRC <e>1121144832 SRC devuan:/mnt/memdisk# dar -c full -R SRC -z6 -q devuan:/mnt/memdisk# cd SRC/linux-5.9.2/ devuan:/mnt/memdisk/SRC/linux-5.9.2# mv Documentation/ doc devuan:/mnt/memdisk/SRC/linux-5.9.2# cd ../.. devuan:/mnt/memdisk# dar -c diff -A full -R SRC -z6 -q devuan:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root <e>17858927 Nov 1 18:18 diff.1.dar -rw-r--r-- 1 root root <e>219047658 Nov 1 18:14 full.1.dar devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# dar -x full -R DST -q devuan:/mnt/memdisk# dar -x diff -R DST -q -w devuan:/mnt/memdisk# diff -r SRC DST && echo "same data" || echo "different data" same data devuan:/mnt/memdisk#`

We can see that the restoration of the full and differential backup over it lead to the exact same directory tree as the source saved files.

##### Rsync

`devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# rsync -arHAXz --delete --info=stats SRC/* BACKUP <e>sent 214,380,105 bytes received 1,359,591 bytes 9,180,412.60 bytes/sec total size is 954,869,250 speedup is 4.43 devuan:/mnt/memdisk# cd SRC/linux-5.9.2/ devuan:/mnt/memdisk/SRC/linux-5.9.2# mv Documentation/ doc devuan:/mnt/memdisk/SRC/linux-5.9.2# cd ../.. devuan:/mnt/memdisk# rsync -arHAXz --delete --info=stats SRC/* BACKUP <e>sent 12,923,292 bytes received 680,190 bytes 3,886,709.14 bytes/sec total size is 954,869,250 speedup is 70.19 devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# rsync -arHAXz --delete --info=stats BACKUP/* DST sent 214,371,610 bytes received 1,359,603 bytes 9,180,051.62 bytes/sec total size is 954,869,250 speedup is 4.43 devuan:/mnt/memdisk#`

We see that after the modification the amount of data pushed to the backup by rsync passes from 214 MiB to only 12 MiB we can consider this as a differential backup, thus this part of the multi-level backup aspect is addressed, but we have lost the access to the first backup: it has been overwritten by the new one, so we lose history but that's a different feature.

##### Tar

`devuan:/mnt/memdisk# tar --listed-incremental=snapshot.file -czf full.tar.gz SRC devuan:/mnt/memdisk# cd SRC/linux-5.9.2/ devuan:/mnt/memdisk/SRC/linux-5.9.2# mv Documentation/ doc devuan:/mnt/memdisk/SRC/linux-5.9.2# cd ../.. devuan:/mnt/memdisk# tar --listed-incremental=snapshot.file -czf diff.tar.gz SRC devuan:/mnt/memdisk# ls -l total 190488 drwxr-xr-x 3 root root 60 Oct 31 19:37 SRC -rw-r--r-- 1 root root <e>9654445 Oct 31 19:49 diff.tar.gz -rw-r--r-- 1 root root <e>184036391 Oct 31 19:49 full.tar.gz -rw-r--r-- 1 root root <e>1361962 Oct 31 19:49 snapshot.file devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# tar --listed-incremental=/dev/null -xf ../full.tar.gz devuan:/mnt/memdisk/DST# tar --listed-incremental=/dev/null -xf ../diff.tar.gz devuan:/mnt/memdisk/DST# cd .. devuan:/mnt/memdisk# diff -r SRC DST/SRC && echo "same data" || echo "different data" same data devuan:/mnt/memdisk#`

Here too, we got the exact same directory as original and modified data

#### Binary Delta

To evaluate the ability the support for binary delta, we will make a first backup of a Debian ISO image, of which we will modify one bit using the bitflip script, then make a differential backup of it. We expect to see the differential backup not resaving the whole file, and though the restoration of the full and differential backup matching the modified file.

##### Dar

`devuan:/mnt/memdisk# dar -c full -z6 -R SRC --delta sig -q devuan:/mnt/memdisk# ./bitflip 100000 SRC/debian-10.6.0-amd64-DVD-2.iso devuan:/mnt/memdisk# dar -c diff -A full -z6 -R SRC -q devuan:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root <e>643 Nov 1 19:45 diff.1.dar -rw-r--r-- 1 root root <e>4704429776 Nov 1 19:05 full.1.dar devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# dar -x full -R DST -q devuan:/mnt/memdisk# dar -x diff -R DST -q devuan:/mnt/memdisk# diff -s SRC/debian-10.6.0-amd64-DVD-2.iso DST/debian-10.6.0-amd64-DVD-2.iso Files SRC/debian-10.6.0-amd64-DVD-2.iso and DST/debian-10.6.0-amd64-DVD-2.iso are identical devuan:/mnt/memdisk#`

For dar the backup we used:

- 4.3 GiB for the full backup (compression ratio of 0,21 %)
- 614 bytes for the differential backup (compression ratio of 99,99998%)

##### Rsync

`devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# rsync -arHAX --info=stats SRC/* DST <e>sent 4,688,066,109 bytes received 35 bytes 284,125,220.85 bytes/sec total size is 4,686,921,728 speedup is 1.00 devuan:/mnt/memdisk# ./bitflip 100000 SRC/debian-10.6.0-amd64-DVD-2.iso devuan:/mnt/memdisk# rsync -arHAX --info=stats SRC/* DST <e>sent 4,688,066,109 bytes received 35 bytes 302,455,880.26 bytes/sec total size is 4,686,921,728 speedup is 1.00 devuan:/mnt/memdisk# rsync -arHAXt --info=stats --no-whole-file SRC/* DST <e>sent 342,469 bytes received 547,803 bytes 30,178.71 bytes/sec total size is 4,686,921,728 speedup is 5,264.60 devuan:/mnt/memdisk# diff -s SRC/debian-10.6.0-amd64-DVD-2.iso DST/debian-10.6.0-amd64-DVD-2.iso Files SRC/debian-10.6.0-amd64-DVD-2.iso and DST/debian-10.6.0-amd64-DVD-2.iso are identical devuan:/mnt/memdisk#`

We had to use `--no-whole-file` to see the binary delta in action with rsync. This feature is not activated when copying on local disk as it does not makes sense (for rsync) because the computation time needed for the binary delta takes more time the the byte to byte copy and because rsync does not store just the delta (no backup history) but modifies the existing backup. Anyway, binary delta is supported (of course!) by rsync.

##### Tar

`devuan:/mnt/memdisk# tar --listed-incremental=snapshot.file -czf full.tar.gz SRC devuan:/mnt/memdisk# ./bitflip 100000 SRC/debian-10.6.0-amd64-DVD-2.iso devuan:/mnt/memdisk# tar --listed-incremental=snapshot.file -czf diff.tar.gz SRC devuan:/mnt/memdisk# ls -l total 9133304 drwxr-xr-x 2 root root 40 Oct 31 17:31 SRC -rwxr--r-- 1 root root 460 Oct 31 16:34 bitflip -rw-r--r-- 1 root root 4676243904 Oct 31 17:28 diff.tar.gz -rw-r--r-- 1 root root <e>4676244172 Oct 31 17:24 full.tar.gz -rw-r--r-- 1 root root 107 Oct 31 17:28 snapshot.file devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# tar --listed-incremental=/dev/null -xf ../full.tar.gz devuan:/mnt/memdisk/DST# tar --listed-incremental=/dev/null -xf ../diff.tar.gz devuan:/mnt/memdisk/DST# diff -s ../SRC/debian-10.6.0-amd64-DVD-2.iso SRC/debian-10.6.0-amd64-DVD-2.iso Files ../SRC/debian-10.6.0-amd64-DVD-2.iso and SRC/debian-10.6.0-amd64-DVD-2.iso are identical devuan:/mnt/memdisk/DST#`

For tar the backup used:

- 4.3 GiB for full backup (compression ratio of 0,22 %)
- 4.3 GiB for the differential backup (compression ratio of 0,22 %)

Binary delta is not supported by tar

#### Detection suspicious modifications

For this test we will use the hide_change script that rely on the bitflip script seen above and try to hide the modifications performed, as a virus, keylogger or rootkit would tend to do. We will make a full backup before the modification and a differential backup after, then observe the behavior.

Here follows the script in action, we see no change using `ls -l` while `stat` shows the exact same information:

- file size
- block used
- Inode number
- permissions
- user and group ownership
- last access time
- last modification time

The only change concerns the inode change time (ctime) that cannot be set manually and signals that some inode properties (but no file content) has changed. This condition occurs, when changing the file permission, ownership, extended attributes and so on, but should not occur when only file's data has changed. `terre:/mnt/memdisk# mkdir SRC terre:/mnt/memdisk# echo "Hello World!" > SRC/file.txt terre:/mnt/memdisk# cat SRC/file.txt Hello World! terre:/mnt/memdisk# ls -l SRC/file.txt -rw-r--r-- 1 root root 13 Nov 12 13:13 SRC/file.txt terre:/mnt/memdisk# stat SRC/file.txt File: SRC/file.txt Size: 13 Blocks: 8 IO Block: 4096 regular file Device: 1bh/27d Inode: 424690 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2020-11-12 13:13:19.021978762 +0100 Modify: 2020-11-12 13:13:09.213998852 +0100 Change: 2020-11-12 13:13:<e>09.213998852 +0100 Birth: - terre:/mnt/memdisk# ./hide_change SRC/file.txt terre:/mnt/memdisk# ls -l SRC/file.txt -rw-r--r-- 1 root root 13 Nov 12 13:13 SRC/file.txt terre:/mnt/memdisk# stat SRC/file.txt File: SRC/file.txt Size: 13 Blocks: 8 IO Block: 4096 regular file Device: 1bh/27d Inode: 424690 Links: 1 Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2020-11-12 13:13:19.021978762 +0100 Modify: 2020-11-12 13:13:09.213998852 +0100 Change: 2020-11-12 13:13:<e>39.549936636 +0100 Birth: - terre:/mnt/memdisk# cat SRC/file.txt Lello World! terre:/mnt/memdisk#`

##### Dar

`terre:/mnt/memdisk# mkdir SRC terre:/mnt/memdisk# echo "Hello World!" > SRC/file.txt terre:/mnt/memdisk# dar -c full -R SRC -N -------------------------------------------- 1 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 1 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# ./hide_change SRC/file.txt terre:/mnt/memdisk# dar -c diff -A full -R SRC -N -q <e>SECURITY WARNING! SUSPICIOUS FILE /mnt/memdisk/SRC/file.txt: ctime changed since archive of reference was done, while no other inode information changed terre:/mnt/memdisk#`

dar issues a warning because of this suspicious condition. Note that we still have the sane file in the full backup, in case of doubt, we can compare it with this modified version:

`terre:/mnt/memdisk# dar -d full -R SRC -q DIFF /mnt/memdisk/SRC/file.txt: <e>different file data, offset of first difference is: 0 Some file comparisons failed terre:/mnt/memdisk#`

The previous test reports that the first byte to have changed is at offset 0, thus this is not just a metadata change that lead to this warning. We can if necessary restore the sane data from the full backup.

##### Rsync

`terre:/mnt/memdisk# rm -rf SRC terre:/mnt/memdisk# mkdir SRC terre:/mnt/memdisk# echo "Hello World!" > SRC/file.txt terre:/mnt/memdisk# rsync -arvHAX SRC DST sending incremental file list created directory DST SRC/ SRC/file.txt sent 146 bytes received 65 bytes 422.00 bytes/sec total size is 13 speedup is 0.06 terre:/mnt/memdisk# ./hide_change SRC/file.txt terre:/mnt/memdisk# rsync -arvHAX SRC DST sending incremental file list sent 83 bytes received 13 bytes 192.00 bytes/sec total size is 13 speedup is 0.14 terre:/mnt/memdisk# cat SRC/file.txt <e>Lello World! terre:/mnt/memdisk# cat DST/SRC/file.txt Hello World! terre:/mnt/memdisk#`

rsync has not reported the problem, but hopefully it has not synchronized the backup, thus we end in a sane version in the DST backup directory though, as user is not aware of this potential risk, the virus/ransomware can spread silently.

##### Tar

`terre:/mnt/memdisk# rm -rf SRC terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir SRC terre:/mnt/memdisk# echo "Hello World!" > SRC/file.txt terre:/mnt/memdisk# tar --listed-incremental=snapshot.file -cf full.tar SRC terre:/mnt/memdisk# ./hide_change SRC/file.txt terre:/mnt/memdisk# tar --listed-incremental=snapshot.file -cvf diff.tar SRC SRC/ SRC/file.txt terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xf ../full.tar terre:/mnt/memdisk/DST# cat SRC/file.txt <e>Hello World! terre:/mnt/memdisk/DST# tar -xf ../diff.tar terre:/mnt/memdisk/DST# cat SRC/file.txt Lello World! terre:/mnt/memdisk/DST#`

As seen above tar does not see any problem, but the file has been resaved as a whole (while its last modification time was unchanged) which lead to corrupt the new backup with potential harmful data. The good point is that you have still the full backup with the sane data. But at a next backup cycle, as you were not notified of the risk, you will lose it and keep only the corrupted version of this file.

#### Snapshot

##### Dar

`terre:/mnt/memdisk# dar -c full -z6 -R /usr <e>--on-fly-isolate snapshot -------------------------------------------- 267245 inode(s) saved including 23 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 267245 -------------------------------------------- EA saved for 5 inode(s) FSA saved for 237962 inode(s) -------------------------------------------- Now performing on-fly isolation... terre:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root 4006060941 Nov 12 15:34 full.1.dar -rw-r--r-- 1 root root 6662595 Nov 12 15:34 snapshot.1.dar terre:/mnt/memdisk# dar <e>-C recreated_snapshot -A full -z6 -q terre:/mnt/memdisk# ls -al *.dar -rw-r--r-- 1 root root 4006060941 Nov 12 15:34 full.1.dar -rw-r--r-- 1 root root 7907094 Nov 12 16:33 recreated_snapshot.1.dar -rw-r--r-- 1 root root 6662595 Nov 12 15:34 snapshot.1.dar terre:/mnt/memdisk# dar -c diff -A snapshot -R /usr -z -------------------------------------------- 23 inode(s) saved including 23 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed <e>267222 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 267245 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# terre:/mnt/memdisk# ls -lh *.dar -rw-r--r-- 1 root root 25M Nov 12 16:37 diff.1.dar -rw-r--r-- 1 root root 3.8G Nov 12 15:34 full.1.dar -rw-r--r-- 1 root root 7.6M Nov 12 16:33 recreated_snapshot.1.dar -rw-r--r-- 1 root root 6.4M Nov 12 15:34 snapshot.1.dar terre:/mnt/memdisk# dar -c diff2 -A recreated_snapshot -R /usr -z -------------------------------------------- 23 inode(s) saved including 23 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed <e>267222 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 267245 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# dar -c snapshot_alone <e>-A + -R /usr -z -------------------------------------------- 23 inode(s) saved including 23 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 267222 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 267245 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# <e>touch /usr/local/src terre:/mnt/memdisk# dar -c faked_diff -A snapshot -R /usr --dry-run -q -vt <e>Adding folder to archive: /usr/local/src Saving Filesystem Specific Attributes for /usr/local/src terre:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root 25537139 Nov 12 16:37 diff.1.dar -rw-r--r-- 1 root root 25537139 Nov 12 16:39 diff2.1.dar -rw-r--r-- 1 root root 4006060941 Nov 12 15:34 full.1.dar -rw-r--r-- 1 root root 7907094 Nov 12 16:33 recreated_snapshot.1.dar -rw-r--r-- 1 root root 6662595 Nov 12 15:34 snapshot.1.dar -rw-r--r-- 1 root root 25537142 Nov 12 16:44 snapshot_alone.1.dar terre:/mnt/memdisk#`

As seen above,a snapshot can be created:

- as part of a backup process (full, differential, incremental or even decremental backup) (`--on-fly-isolate`)
- from a existing backup (`-C`)
- alone by a dedicated operation (`-A +`)

`root@terre:/mnt/memdisk# ls -l full.1.dar -rw-r--r-- 1 root root <e>3895581703 Nov 29 21:53 full.1.dar root@terre:/mnt/memdisk# dar -l full -q FATAL error, aborting operation: Cannot open catalogue: unknown compression root@terre:/mnt/memdisk# !bitflip bitflip 31124653000 full.1.dar root@terre:/mnt/memdisk# dar -l full -q Archive version format : 11 Compression algorithm used : gzip Compression block size used : 0 Symmetric key encryption used : none Asymmetric key encryption used : none Archive is signed : no Sequential reading marks : present User comment : N/A <e>Catalogue size in archive : 7799028 bytes Archive is composed of 1 file(s) File size: 3895581703 bytes The global data compression ratio is: 51% CATALOGUE CONTENTS : total number of inode : 263480 fully saved : 263480 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 18080 - plain files : 216142 - symbolic links : 29258 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 11 - number of reference to hard linked inodes: 34 destroyed entries information 0 file(s) have been record as destroyed since backup of reference root@terre:/mnt/memdisk# <e>bitflip 31124653000 full.1.dar root@terre:/mnt/memdisk# dar -t full Final memory cleanup... FATAL error, aborting operation: Cannot open catalogue: unknown compression root@terre:/mnt/memdisk# dar -t full <e>-A snapshot -------------------------------------------- 263503 item(s) treated 0 item(s) with error 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 263503 -------------------------------------------- root@terre:/mnt/memdisk# root@terre:/mnt/memdisk# dar -t full <e>--sequential-read A problem occurred while reading this archive contents: <e>Cannot open catalogue: unknown compression -------------------------------------------- 263503 item(s) treated 0 item(s) with error 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 263503 -------------------------------------------- root@terre:/mnt/memdisk#`

Once created a snapshot can be used:

- to create a differential or incremental backup
- to list the files that would be saved (thus which have changed, were added or have been removed) thus all changes since the time the snapshot was made (using the `--dry-run` option)
- rescue a corrupted backup when the corruption falled into the backup table of content located at the end of the dar backup (only if it has been created based on the backup to rescue either using `-C option` maybe long after the backup was made or using `--on-fly-isolate` at the same time the backup was created.

Note, as shown above, a table of content (aka "catalogue") corruption, can also partially be recovered using the `--sequential-read` mode, it will just not let dar remove files that were removed since the reference backup was made (this does thus not concern full backups, as here).

##### Rsync

This feature is not supported by rsync.

##### Tar

tar can generate snapshot:

- alone redirecting the backup output to /dev/null
- as part of a backup process (in fact tar cannot do else)

`terre:/mnt/memdisk# tar --listed-incremental=snapshot.file -czf full.tar.gz /usr tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets terre:/mnt/memdisk# ls -l snapshot.file -rw-r--r-- 1 root root 6288644 Nov 12 15:12 snapshot.file terre:/mnt/memdisk# cp snapshot.file snapshot.file.ref tar --listed-incremental=snapshot.file -cvf /dev/null /usr /usr/ /usr/bin/ /usr/games/ /usr/include/ /usr/include/X11/ /usr/include/X11/bitmaps/ /usr/include/arpa/ /usr/include/asm-generic/ /usr/include/attr/ /usr/include/c++/ /usr/include/c++/ <e>[...] /usr/share/zoneinfo/right/Canada/ /usr/share/zoneinfo/right/Chile/ /usr/share/zoneinfo/right/Etc/ /usr/share/zoneinfo/right/Europe/ /usr/share/zoneinfo/right/Indian/ /usr/share/zoneinfo/right/Mexico/ /usr/share/zoneinfo/right/Pacific/ /usr/share/zoneinfo/right/SystemV/ /usr/share/zoneinfo/right/US/ /usr/share/zsh/ /usr/share/zsh/site-functions/ /usr/share/zsh/vendor-completions/ /usr/src/ terre:/mnt/memdisk# ls -l sna snapshot.file snapshot.file.ref terre:/mnt/memdisk# ls -l snapshot.file* -rw-r--r-- 1 root root 6288644 Nov 12 15:20 snapshot.file -rw-r--r-- 1 root root 6288644 Nov 12 15:18 snapshot.file.ref terre:/mnt/memdisk#`

If a snapshot can be used (and is in fact required) to make a differential backup, it cannot really be used to see the difference a current living filesystem has with a given snapshot. Worse, doing so modifies the snapshot, so you have first to make a copy to not screw up your backup process. Worse, if incremental backup fails and you have not created a copy of the backup, your snapshot being modified you will mostly have to remake the whole backup process from the full backup to be sure to not miss backing up some modified files. Same thing if you lose by mistake the snapshot file.

#### On-fly hashing

##### Dar

`terre:/mnt/memdisk# dar -c backup -R /usr -g usr/bin -z6 <e>--hash sha1 -------------------------------------------- 0 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 8 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 8 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# ls -l *.dar* -rw-r--r-- 1 root root 171 Nov 12 17:22 backup.1.dar -rw-r--r-- 1 root root 55 Nov 12 17:22 <e>backup.1.dar.sha1 terre:/mnt/memdisk# sha1sum -c backup.1.dar.sha1 <e>backup.1.dar: OK terre:/mnt/memdisk#`

##### Rsync

not supported by rsync

##### Tar

not supported by tar

#### Custom command during operation

As an example (but there is much more thing that can be done), we take the case of a automounted directory. Such type of volume is mounted only when used, if not used no mount point directory shows and unless you know it exists, no backup of its content is performed. The idea, is when entering the parent directory at backup process to trigger the mount point for the backup to include them.

##### Dar

`terre:/mnt/memdisk# cat /etc/auto.mnt Espace -defaults,relatime,acl,bg,rsize=8192,wsize=8192 nfs.systeme-solaire.espace:/mnt/Externe/Espace Commun -defaults,relatime,acl,bg,rsize=8192,wsize=8192,ro nfs.systeme-solaire.espace:/mnt/Externe/Commun Backup -defaults,relatime,acl,bg,rsize=8192,wsize=8192,ro nfs.systeme-solaire.espace:/mnt/Backup terre:/mnt/memdisk# ls -l /mnt/Externe/ total 4 drwxr-xr-x 7 root root 4096 Jul 14 17:58 Espace terre:/mnt/memdisk# dar -c backup -R / -g /mnt -q terre:/mnt/memdisk# dar -l backup [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ ][ ] drwxr-xr-x 0 0 0 Wed Oct 21 18:17:07 2020 mnt [Saved][-] [-L-][ ][ ] drwxr-xr-x 1000 1002 0 Mon Nov 9 11:56:54 2020 mnt/localdisk [Saved][-] [---][-----][ ] lrwxrwxrwx 0 0 0 Thu Aug 15 23:29:46 2019 mnt/Backup [Saved][-] [---][ ][ ] drwxr-xr-x 0 0 0 Thu Nov 12 17:42:11 2020 mnt/Externe [Saved][-][Saved][---][ ][ ] drwxr-xr-x 0 0 0 Tue Jul 14 17:58:57 2020 mnt/Externe/Espace terre:/mnt/memdisk# terre:/mnt/memdisk# rm backup.1.dar terre:/mnt/memdisk# dar -c backup -R / -g mnt -q '-<' mnt '-=' 'file %p/Externe/Backup %p/Externe/Commun' /mnt/Externe/Backup: directory /mnt/Externe/Commun: directory terre:/mnt/memdisk# dar -l backup [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ ][ ] drwxr-xr-x 0 0 0 Wed Oct 21 18:17:07 2020 mnt [Saved][-] [-L-][ ][ ] drwxr-xr-x 1000 1002 0 Mon Nov 9 11:56:54 2020 mnt/localdisk [Saved][-] [---][-----][ ] lrwxrwxrwx 0 0 0 Thu Aug 15 23:29:46 2019 mnt/Backup [Saved][-] [---][ ][ ] drwxr-xr-x 0 0 0 Thu Nov 12 18:01:41 2020 mnt/Externe [Saved][-][Saved][---][ ][ ] drwxr-x--- 993 1002 0 Wed Nov 11 10:21:55 2015 <e>mnt/Externe/Commun [Saved][-][Saved][---][ ][ ] drwxr-xr-x 0 0 0 Sun Sep 13 12:22:24 2020 <e>mnt/Externe/Backup [Saved][-][Saved][---][ ][ ] drwxr-xr-x 0 0 0 Tue Jul 14 17:58:57 2020 mnt/Externe/Espace terre:/mnt/memdisk# ls -l /mnt/Externe/ total 12 drwxr-xr-x 9 root root 4096 Sep 13 12:22 Backup drwxr-x--- 4 commun maison 4096 Nov 11 2015 Commun drwxr-xr-x 7 root root 4096 Jul 14 17:58 Espace terre:/mnt/memdisk#`

In the previous example we see that the /mnt/Externe directory is a mount point containing three auto-mounted volumes: `Espace`, `Commun` and `Backup`. At first only `Espace` was mounted. Performing a backup without care will skip the two other directories.

In a second time, thanks to the `-<` and `-=` options, we instructed dar to run the `file` command on the two missing directories when entering `/mnt`. As a result, we now see both of them in the backup. We could do that before executing the backup, but as the backup may include many other directories the time between such operation done before starting the backup and the time the backup finally saves the automount point at `/mnt/Externe` may exceed the automount timeout leading them to be unmounted and disappear before the backup process reaches them.

`terre:/mnt/memdisk# dar -c backup -R / -g usr/bin --hash sha512 -s 100M -q terre:/mnt/memdisk# ls -l backup.* -rw-r--r-- 1 root root 104857600 Nov 12 18:30 backup.1.dar -rw-r--r-- 1 root root 143 Nov 12 18:30 backup.1.dar.sha512 -rw-r--r-- 1 root root 104857600 Nov 12 18:30 backup.2.dar -rw-r--r-- 1 root root 143 Nov 12 18:30 backup.2.dar.sha512 -rw-r--r-- 1 root root 104857600 Nov 12 18:30 backup.3.dar -rw-r--r-- 1 root root 143 Nov 12 18:30 backup.3.dar.sha512 -rw-r--r-- 1 root root 63577207 Nov 12 18:30 backup.4.dar -rw-r--r-- 1 root root 143 Nov 12 18:30 backup.4.dar.sha512 terre:/mnt/memdisk# dar -t backup <e>-E 'sha512sum -c %p/%b.%N.%e.sha512' backup.4.dar: OK backup.1.dar: OK backup.2.dar: OK backup.3.dar: OK backup.4.dar: OK -------------------------------------------- 2594 item(s) treated 0 item(s) with error 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 2594 -------------------------------------------- terre:/mnt/memdisk#`

In this example, this we used slicing with on-fly hashing which generated for each slice the corresponding sha512 hash file. Then we tested the archive content and at the same time the hash files thanks to the `-E` option. Of course any user command or shell or python script, can be used instead, and for backup, restoration, testing, snashotting,...

##### Rsync

not supported by rsync

##### Tar

tar has the `-F option` to launch a command after each tape, but it is only available with multi-volume tar archive, which in turn cannot be used with compression. Thus we won't test it, as it is quite restrictive and does not match any common use cases.

#### Dry-run execution

##### Dar

`terre:/mnt/memdisk/A# ls -l <e>total 0 terre:/mnt/memdisk/A# dar -c backup -R / -g usr/bin --dry-run -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 34 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2628 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2154 inode(s) -------------------------------------------- terre:/mnt/memdisk/A# ls -l <e>total 0 terre:/mnt/memdisk/A#`

##### Rsync

`terre:/mnt/memdisk# rsync -arHAX --dry-run /usr/bin DST terre:/mnt/memdisk# ls -l DST <e>ls: cannot access 'DST': No such file or directory terre:/mnt/memdisk#`

##### Tar

does not seem supported by tar

#### User message within backup

##### Dar

`terre:/mnt/memdisk# dar -c backup <e>--user-comment "passphrase is the usual one. Archive was made on %d on host %h" -R / -g usr/bin -K camellia: -zxz -s 100M Archive backup requires a password: Please confirm your password: -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 34 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2628 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2154 inode(s) -------------------------------------------- terre:/mnt/memdisk# dar -l backup <e>-aheader Archive version format : 11 Compression algorithm used : xz Compression block size used : 0 Symmetric key encryption used : camellia 256 Asymmetric key encryption used : none Archive is signed : no Sequential reading marks : present User comment : <e>passphrase is the usual one. Archive was made on Thu Nov 12 18:57:35 2020 on host terre KDF iteration count : 10000 KDF hash algorithm : argon2 Salt size : 32 bytes Final memory cleanup... FATAL error, aborting operation: header only mode asked terre:/mnt/memdisk#`

The use of the `-aheader` let one see the archive header that is always in clear-text. The usual listing operation provides some additional informations from the ciphered table of content and thus in that context requires the passphrase:

`terre:/mnt/memdisk# dar <e>-l backup <e>-q <e>Archive backup requires a password: Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted Archive version format : 11 Compression algorithm used : xz Compression block size used : 0 Symmetric key encryption used : camellia 256 Asymmetric key encryption used : none Archive is signed : no Sequential reading marks : present User comment : <e>passphrase is the usual one. Archive was made on Thu Nov 12 18:57:35 2020 on host terre KDF iteration count : 10000 KDF hash algorithm : argon2 Salt size : 32 bytes Catalogue size in archive : 78268 bytes Archive is composed of 2 file(s) File size : 104857600 bytes Last file size : 17168696 bytes Archive total size is : 122026296 bytes <e>The global data compression ratio is: 72% <e>CATALOGUE CONTENTS : total number of inode : 2589 fully saved : 2589 binay delta patch : 0 inode metadata only : 0 distribution of inode(s) - directories : 2 - plain files : 2152 - symbolic links : 435 - named pipes : 0 - unix sockets : 0 - character devices : 0 - block devices : 0 - Door entries : 0 hard links information - number of inode with hard link : 5 - number of reference to hard linked inodes: 10 destroyed entries information 0 file(s) have been record as destroyed since backup of reference terre:/mnt/memdisk#`

##### Rsync

not supported by rsync

##### Tar

not supported by tar

#### backup sanity test

##### Dar

`terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -zlz4 -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 34 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2628 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2154 inode(s) -------------------------------------------- terre:/mnt/memdisk# dar -t backup -------------------------------------------- 2594 item(s) treated 0 item(s) with error 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 2594 -------------------------------------------- terre:/mnt/memdisk#`

##### Rsync

It does not seems possible to let rsync check that the target or destination directory is sane and usuable. All operation modify the destination file or save modified files in either the destination directory (the backup) or an alternate directory (`--compare-dest` option).

##### Tar

`terre:/mnt/memdisk# rm -rf backup.tar.gz terre:/mnt/memdisk# tar -czf backup.tar.gz /usr/bin tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets terre:/mnt/memdisk# tar -tzf backup.tar.gz usr/bin/ usr/bin/bitmap usr/bin/dot usr/bin/indi_usbdewpoint usr/bin/ruby2.5 usr/bin/pod2man usr/bin/iptables-xml usr/bin/knotify4 usr/bin/fakeroot usr/bin/xclock <e>[...] /bin/traceproto usr/bin/ofm2opl usr/bin/akonadi_archivemail_agent usr/bin/resizecons usr/bin/rletopnm usr/bin/dh_install usr/bin/updvitomp usr/bin/h2xs usr/bin/xmessage terre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

#### Comparing with original data

##### Dar

`terre:/mnt/memdisk# dar -c backup -R SRC -q terre:/mnt/memdisk# dar -d backup -R SRC -------------------------------------------- 2594 item(s) treated 0 item(s) do not match those on filesystem 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 2594 -------------------------------------------- terre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

##### Rsync

Does not seems supported by rsync

##### Tar

`terre:/mnt/memdisk/SRC# tar -czf ../backup.tar.gz . terre:/mnt/memdisk/SRC# tar -dzf ../backup.tar.gz terre:/mnt/memdisk/SRC# echo $? 0 terre:/mnt/memdisk/SRC#`

#### Tunable verbosity

##### Dar

`terre:/mnt/memdisk# dar -c backup -R / -g usr/bin <e>-q terre:/mnt/memdisk# rm backup.1.dar terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 34 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2628 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2154 inode(s) -------------------------------------------- terre:/mnt/memdisk# dar -c backup -R / -g usr/bin <e>-vm Arguments read from /usr/local/etc/darrc : Creating low layer: Writing archive into a plain file object... Adding a new layer on top: Caching layer for better performances... Writing down the archive header... Adding a new layer on top: Escape layer to allow sequential reading... All layers have been created successfully Building the catalog object... Processing files for backup... Writing down archive contents... Closing the escape layer... Writing down the first archive terminator... Writing down archive trailer... Writing down the second archive terminator... Closing archive low layer... Archive is closed. -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 34 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2628 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2154 inode(s) -------------------------------------------- Making room in memory (releasing memory used by archive of reference)... Final memory cleanup... terre:/mnt/memdisk# rm -f backup* terre:/mnt/memdisk# dar -c backup -R / -g usr/bin <e>-vt -q Adding folder to archive: /usr Saving Filesystem Specific Attributes for /usr Adding folder to archive: /usr/bin Saving Filesystem Specific Attributes for /usr/bin Adding file to archive: /usr/bin/bitmap Saving Filesystem Specific Attributes for /usr/bin/bitmap <e>[...] Saving Filesystem Specific Attributes for /usr/bin/dh_install Adding symlink to archive: /usr/bin/updvitomp Adding file to archive: /usr/bin/h2xs Saving Filesystem Specific Attributes for /usr/bin/h2xs Adding file to archive: /usr/bin/xmessage Saving Filesystem Specific Attributes for /usr/bin/xmessage terre:/mnt/memdisk# rm -f backup* terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -vd Inspecting directory /root Inspecting directory /bin Inspecting directory /sbin Inspecting directory /tmp Inspecting directory /sys Inspecting directory /lib <e>[...] Inspecting directory /var Inspecting directory /proc Inspecting directory /dev Inspecting directory /etc Inspecting directory /media Inspecting directory /run terre:/mnt/memdisk# terre:/mnt/memdisk# rm -f backup.* terre:/mnt/memdisk# dar -c backup -R / -g usr/bin <e>-vf -q Finished Inspecting directory /usr/bin , saved 408 Mio, compression ratio 13% Finished Inspecting directory /usr , saved 408 Mio, compression ratio 13% terre:/mnt/memdisk# terre:/mnt/memdisk# dar -c backup -R / -g usr/bin <e>-vmasks -q directory tree filter: AND | OR | | Is subdir of: /usr/bin [case sensitive] | +-- +-- filename filter: AND | TRUE +-- EA filter: AND | TRUE +-- Compression filter: TRUE terre:/mnt/memdisk#`

dar has several options to define which type of message to show or not to show: `-v, -vs, -vt, -vd, -vf, -vm, -vmasks, -q`. They can be combined.

##### Rsync

`terre:/mnt/memdisk# rsync -arHAX /usr/bin DST terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# rsync -arHAX <e>-v /usr/bin DST sending incremental file list created directory DST bin/ bin/2to3-2.7 bin/411toppm bin/7z bin/7za bin/7zr bin/FvwmCommand <e>[...] bin/zstdmt -> zstd bin/perl => bin/perl5.28.1 bin/perlbug => bin/perlthanks bin/python3.7 => bin/python3.7m bin/pkg-config => bin/x86_64-pc-linux-gnu-pkg-config bin/unzip => bin/zipinfo sent 437,298,617 bytes received 42,381 bytes 174,936,399.20 bytes/sec total size is 445,394,557 speedup is 1.02 root@terre:/mnt/memdisk# rsync -arHAX <e>--info=progress2 /usr/bin DST 437,083,500 98% 128.42MB/s 0:00:03 (xfr#2152, to-chk=0/2593) root@terre:/mnt/memdisk#`

`-v option` leads to a more verbose output, while `-q` remove the non error messages. Using both at the same time seems not to be different than using `-q` alone. However rsync has a very rich set of additional options like `--info`, `--debug` that can be added on top.

##### Tar

`terre:/mnt/memdisk# tar -czf backup.tar.gz /usr/bin tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets terre:/mnt/memdisk# rm backup.tar.gz terre:/mnt/memdisk# tar <e>-v -czf backup.tar.gz /usr/bin tar: Removing leading `/' from member names /usr/bin/ /usr/bin/bitmap /usr/bin/dot /usr/bin/indi_usbdewpoint /usr/bin/ruby2.5 /usr/bin/pod2man /usr/bin/iptables-xml /usr/bin/knotify4 <e>[...] /usr/bin/traceproto /usr/bin/ofm2opl /usr/bin/akonadi_archivemail_agent /usr/bin/resizecons /usr/bin/rletopnm /usr/bin/dh_install /usr/bin/updvitomp /usr/bin/h2xs /usr/bin/xmessage terre:/mnt/memdisk#`

tar only provides the `-v` option to increase verbosity.

#### Modify Backup content

We will perform two types of tests:

- remove one or several files from an existing backup without having to make a new backup process
- add some forgotten files to an existing backup without performing a new full backup process

##### Dar

`terre:/mnt/memdisk# dar -c backup -R / -g usr/bin -z6 -q terre:/mnt/memdisk# dar -l backup -g usr/bin/emacs-gtk [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ 64%][ ] drwxr-xr-x root root 408 Mio Sun Jun 2 23:25:09 2019 usr [Saved][-] [-L-][ 64%][ ] drwxr-xr-x root root 408 Mio Sun Nov 8 13:43:58 2020 usr/bin [Saved][ ] [-L-][ 90%][X] -rwxr-xr-x root root 38 Mio Thu Sep 5 04:35:24 2019 <e>usr/bin/emacs-gtk terre:/mnt/memdisk# dar -A backup <e>-+ without-emacs -ak <e>-P usr/bin/emacs-gtk -vs -q Skipping file: <ROOT>/usr/bin/emacs-gtk terre:/mnt/memdisk# dar -l without-emacs -g usr/bin/emacs-gtk [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ 62%][ ] drwxr-xr-x root root 370 Mio Sun Jun 2 23:25:09 2019 usr [Saved][-] [-L-][ 62%][ ] drwxr-xr-x root root 370 Mio Sun Nov 8 13:43:58 2020 usr/bin terre:/mnt/memdisk#rm backup.* terre:/mnt/memdisk#mv without-emacs.1.dar backup.1.dar terre:/mnt/memdisk#`

dar does not modify a existing backup but creates a copy of it with the requested files or directory removed. The process can be quick even with compression thanks to the `-ak` option that avoid uncompressing and recompressing file that are kept. Before removing the old backup you can test the sanity of the new generated one.

`terre:/mnt/memdisk# dar -c emacs -R / -g usr/bin/emacs-gtk -z6 -q terre:/mnt/memdisk# dar -l emacs [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ 90%][ ] drwxr-xr-x root root 38 Mio Sun Jun 2 23:25:09 2019 usr [Saved][-] [-L-][ 90%][ ] drwxr-xr-x root root 38 Mio Sun Nov 8 13:43:58 2020 usr/bin [Saved][ ] [-L-][ 90%][X] -rwxr-xr-x root root 38 Mio Thu Sep 5 04:35:24 2019 usr/bin/emacs-gtk terre:/mnt/memdisk# <e>dar -A backup -@ emacs -+ with-emacs -ak -------------------------------------------- 2594 inode(s) added to archive with 10 hard link(s) recorded 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted -------------------------------------------- EA saved for 3 inode(s) FSA saved for 2159 inode(s) -------------------------------------------- Total number of inode(s) considered: 2594 -------------------------------------------- terre:/mnt/memdisk# dar -l with-emacs -g usr/bin/emacs-gtk [Data ][D][ EA ][FSA][Compr][S]| Permission | User | Group | Size | Date | filename --------------------------------+------------+-------+-------+---------+-------------------------------+------------ [Saved][-] [-L-][ 64%][ ] drwxr-xr-x root root 408 Mio Sun Jun 2 23:25:09 2019 usr [Saved][-] [-L-][ 64%][ ] drwxr-xr-x root root 408 Mio Sun Nov 8 13:43:58 2020 usr/bin [Saved][ ] [-L-][ 90%][X] -rwxr-xr-x root root 38 Mio Thu Sep 5 04:35:24 2019 usr/bin/emacs-gtk terre:/mnt/memdisk# rm emacs.* backup.* terre:/mnt/memdisk# mv with-emacs.1.dar backup.1.dar terre:/mnt/memdisk#`

Here to add files to a existing backup we must make a small backup of these files only, then merge this backup with the backup we want to modify. Nothing of the source data is touched in this operation, is something goes wrong or if you made an error, you can fix and restart without taking the risk to lose data.

##### Rsync

The backup made by rsync is just a copy of the save files, removing a file from the backup is as simple as calling `rm` on that file in the repository that is considered the backup.

While adding a new file in the backup can be done by using rsync as usual including the directory tree where this file resides.

##### Tar

`terre:/mnt/memdisk# tar -czf backup.tar.gz /usr/bin tar: Removing leading `/' from member names tar: Removing leading `/' from hard link targets terre:/mnt/memdisk# tar -tvf backup.tar.gz | grep emacs-gtk -rwxr-xr-x root/root 39926024 2019-09-05 04:35 usr/bin/emacs-gtk terre:/mnt/memdisk# tar -tvf backup.tar.gz | grep emacs-gtk -rwxr-xr-x root/root 39926024 2019-09-05 04:35 usr/bin/emacs-gtk terre:/mnt/memdisk# tar --delete usr/bin/emacs-gtk -f backup.tar.gz tar: Cannot update compressed archives tar: Error is not recoverable: exiting now terre:/mnt/memdisk#`

Well, tar cannot manipulate compressed archives. What the point then to remove a file from a backup if storage space is not an issue, else, would compression be used?

#### stdin/stdout backup read/write

##### Dar

`terre:/mnt/memdisk# dar -c - -z6 -R SRC > backup.file -------------------------------------------- 2594 inode(s) saved including 5 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2594 -------------------------------------------- EA saved for 3 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# dar -x - --sequential-read -R DST < backup.file -------------------------------------------- 2594 inode(s) restored including 5 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 2594 -------------------------------------------- EA restored for 3 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk#`

dar can read a backup from stdin and write a backup to stdout.

##### Rsync

Using stdin/stdout to send to or read from backed up data does not seems possible with rsync

##### Tar

`terre:/mnt/memdisk# tar -czf - SRC > backup.file terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xzf - < ../backup.file terre:/mnt/memdisk/DST#`

tar can read a backup from stdin and write a backup to stdout.

#### Remote network storage

For Remote Network storage, if you use a personal NAS you may avoid generating ciphered backup, though you still should transfer it using a secured protocol if the underlaying network is not your own from end to end (for example a part of the path goes over Internet without IPSec or equivalent).

Why ciphering backup if using secure transfer protocol?

- Secure transfer avoids one to read your data in transit in particular your credentials to the remote site, but at the end the transferred data is stored in clear at the other end.
- ciphered data avoids the owner of the remote storage to access to your data without your consent, though it does not protect one to intercept your transfer and read the credentials you used to connected to the remote storage. This one could then connect later and delete all your backup... which is surely not what you want.

In the following we will use both: secure protocol and ciphered backup, without using local storage. We will also need compression to save precious space (usually you pay for the cloud storage you use) and maybe slicing depending on the constraints imposed by the remote storage (some provider ask you to pay an extra amount to store larger files, having slicing avoids you paying extra cost in such context). Another use case of slicing is when the file transfer protocol is not able to continue an interrupted transfer, you will then only need to restart it for the last slice, not the whole backup.

`terre:/mnt/memdisk# sftp denis@dar The authenticity of host 'dar (192.168.6.32)' can't be established. RSA key fingerprint is SHA256:KN3o/psWC512grcZ5/J5dTSg9PzIXbZAHiig/hqfkc8. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added 'dar,192.168.6.32' (RSA) to the list of known hosts. denis@dar's password: Connected to denis@dar. sftp> bye terre:/mnt/memdisk#`

##### Dar

`terre:/mnt/memdisk# dar -c sftp://denis@dar/home/denis/backup -R / -g etc -K aes: -zlz4 -s 1M Please provide the password for login denis at host dar: <e>Archive backup requires a password: <e>Please confirm your password: -------------------------------------------- 2360 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 27 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 2387 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 1523 inode(s) -------------------------------------------- terre:/mnt/memdisk# sftp denis@dar denis@dar's password: Connected to denis@dar. sftp> ls -l -rw-r--r-- 1 denis denis 1048576 Nov 26 17:15 backup.1.dar -rw-r--r-- 1 denis denis 1048576 Nov 26 17:15 backup.2.dar -rw-r--r-- 1 denis denis 1048576 Nov 26 17:15 backup.3.dar -rw-r--r-- 1 denis denis 474982 Nov 26 17:15 backup.4.dar sftp> bye terre:/mnt/memdisk#`

The backup results in four ciphered slices located on the remote sftp server. Let's add a `-E option` to see which slice are being read while testing the archive.

`terre:/mnt/memdisk# dar -t sftp://denis@dar/home/denis/backup -E "echo 'openning slice %p/%b.%N.%e'" Please provide the password for login denis at host dar: openning slice /home/denis/backup.4.dar <e>Archive backup requires a password: Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted openning slice /home/denis/backup.1.dar openning slice /home/denis/backup.2.dar openning slice /home/denis/backup.3.dar openning slice /home/denis/backup.4.dar -------------------------------------------- 2360 item(s) treated 0 item(s) with error 0 item(s) ignored (excluded by filters) -------------------------------------------- Total number of items considered: 2360 -------------------------------------------- terre:/mnt/memdisk#`

We see that all slices have been read as expected, now let's restore /etc/fstab in the current directory and compare the restored files with the real /etc/fstab

`terre:/mnt/memdisk# dar -x sftp://denis@dar/home/denis/backup -E "echo 'openning slice %p/%b.%N.%e'" -g etc/fstab --flat Please provide the password for login denis at host dar: <e>openning slice /home/denis/backup.4.dar Archive backup requires a password: Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted <e>1 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 269 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 270 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# <e>diff fstab /etc/fstab terre:/mnt/memdisk# echo $? <e>0 terre:/mnt/memdisk#`

As seen above only one slice (slice #4) has been necessary to restore /etc/fstab. But let's save two files, a huge one and a small one into a single sliced backup and measure the transfer time of backup and restoration of this ciphered an compressed backup through sftp. We have added public key authentification for precise time measurement:

`terre:/mnt/memdisk# sftp denis@dar Connected to denis@dar. sftp> bye terre:/mnt/memdisk# ls -l SRC total 315396 -rw------- 1 root root 322961408 Nov 26 17:28 devuan_beowulf_3.0.0_amd64-netinstall.iso -rw-r--r-- 1 root root 994 Nov 26 17:29 fstab terre:/mnt/memdisk# terre:/mnt/memdisk# time dar -c sftp://denis@dar/home/denis/backup -R SRC -z6 -K aes:hello -afile-auth -q 20.769u 2.445s <e>0:22.77 101.8% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# time dar -x sftp://denis@dar/home/denis/backup -R DST -K hello -afile-auth -q Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted 11.826u 4.211s <e>0:15.88 100.9% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk# diff -rs SRC DST <e>Files SRC/devuan_beowulf_3.0.0_amd64-netinstall.iso and DST/devuan_beowulf_3.0.0_amd64-netinstall.iso are identical <e>Files SRC/fstab and DST/fstab are identical terre:/mnt/memdisk# rm DST/fstab terre:/mnt/memdisk# time dar -x sftp://denis@dar/home/denis/backup -R DST -K hello -afile-auth -q -g fstab Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted 0.680u 0.012s <e>0:00.87 79.3% 0+0k 0+0io 0pf+0w terre:/mnt/memdisk#`

While restoring the whole backup needs 15 seconds of transfer time, restoring `fstab` alone requires only 0.87 second, as there is only one slice, this shows that dar is reading only the necessary part of the archive even within a slice to perform the operation.

##### Rsync

`terre:/mnt/memdisk# rsync -arHAXSzq /etc denis@dar:/home/denis terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# rsync -arHAXSzq denis@dar:/home/denis/etc/fstab . terre:/mnt/memdisk# diff fstab /etc/fstab terre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

We can backup to a remote sftp server, we can compress the data on-fly but it is not stored compressed nor ciphered. Restoration operation is possible per file or for the whole backup

##### Tar

tar has no way to perform sftp or other secured transfer protocol, nor encryption by itself. When time comes to restore a particular file, the whole backup has to be retrieved, unciphered and uncompressed to restore even just a sigle file

### Backup Robustness

The objective of this test is to measure the way the backup tools under test behave when the backup has been corrupted. We will here just flip one byte of the backup, at the beginning, in the middle or at the end of the backup and observe the consequences in term of ability to restore the backup.

##### Dar

`terre:/mnt/memdisk# dar -c backup -R SRC -z6 -------------------------------------------- 74725 inode(s) saved including 0 hard link(s) treated 0 inode(s) changed at the moment of the backup and could not be saved properly 0 byte(s) have been wasted in the archive to resave changing files 0 inode(s) with only metadata changed 0 inode(s) not saved (no inode/file change) 0 inode(s) failed to be saved (filesystem error) 0 inode(s) ignored (excluded by filters) 0 inode(s) recorded as deleted from reference backup -------------------------------------------- Total number of inode(s) considered: 74725 -------------------------------------------- EA saved for 0 inode(s) FSA saved for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# ls -al backup* -rw-r--r-- 1 root root 219088536 Nov 17 17:45 backup.1.dar terre:/mnt/memdisk# ./hide_change backup.1.dar 1 terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# dar -x backup -R DST <e>backup.1.dar is not a valid file (wrong magic number), please provide the good file. [return = YES | Esc = NO] Escaping... Final memory cleanup... Aborting program. User refused to continue while asking: backup.1.dar is not a valid file (wrong magic number), please provide the good file. terre:/mnt/memdisk# dar -x backup -R DST <e>-alax LAX MODE: In spite of its name, backup.1.dar does not appear to be a dar slice, assuming a data corruption took place and continuing LAX MODE: Archive is flagged as having escape sequence (which is normal in recent archive versions). However if this is not expected, shall I assume a data corruption occurred in this field and that this flag should be ignored? (If unsure, refuse) [return = YES | Esc = NO] Escaping... -------------------------------------------- <e>74725 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- <e>Total number of inode(s) considered: 74725 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# diff -r SRC DST terre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

Modifying the first bit dar has seen the corruption. We can use the lax mode (`-alax` option) to bypass this corruption and then the restoration proceeds normally. We can try a bit further for example somewhere in the middle of the archive, thus at offset 876354144 (half of the size expressed in bit, not byte):

`terre:/mnt/memdisk# ls -l backup* -rw-r--r-- 1 root root 219088536 Nov 17 17:45 backup.1.dar terre:/mnt/memdisk# hide_change backup.1.dar 876354144 terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# dar -x backup -R DST <e>Error while restoring /mnt/memdisk/DST/linux-5.9.8/drivers/mtd/spi-nor/core.c : compressed data CRC error -------------------------------------------- 74724 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 1 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 74725 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- Final memory cleanup... All files asked could not be restored terre:/mnt/memdisk# diff -rq SRC DST <e>Files SRC/linux-5.9.8/drivers/mtd/spi-nor/core.c and DST/linux-5.9.8/drivers/mtd/spi-nor/core.c differ terre:/mnt/memdisk#`

One file could not be restored properly as reported by dar, but all other files could be and are identical to their respective originals. Let's modifying the last bit for completness:

`terre:/mnt/memdisk# ls -al *.dar -rw-r--r-- 1 root root 219088537 Nov 17 17:45 backup.1.dar terre:/mnt/memdisk# cp backup.1.dar backop.1.dar terre:/mnt/memdisk# hide_change backup.1.dar 1752708287 terre:/mnt/memdisk# ls -al *.dar -rw-r--r-- 1 root root 219088536 Nov 17 17:58 backop.1.dar -rw-r--r-- 1 root root 219088536 Nov 17 17:58 backup.1.dar terre:/mnt/memdisk# diff backup.1.dar backop.1.dar Binary files backup.1.dar and backop.1.dar differ terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# dar -x backup -R DST -------------------------------------------- 74725 inode(s) restored including 0 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 74725 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- terre:/mnt/memdisk# terre:/mnt/memdisk# diff -rq SRC DST terre:/mnt/memdisk# echo $? 0 terre:/mnt/memdisk#`

By chance it did not affected the ability to restore the backup. However if it ever had, we have several fallbacks: the `--sequential-read` mode, the use a already created snapshot (aka isolated catalogue) as seen about the snapshot feature to backup the internal table of content, or as last resort the `-alax option` eventually combined with the `--sequential-read` mode and a backup snapshot.

##### Rsync

`terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# rsync -arHAXS SRC/* DST terre:/mnt/memdisk# ls -al DST total 0 drwxr-xr-x 3 root root 60 Nov 17 18:07 . drwxrwxrwt 4 root root 140 Nov 17 18:07 .. drwxrwxr-x 24 root root 740 Nov 10 21:16 linux-5.9.8 terre:/mnt/memdisk# diff -rq SRC DST terre:/mnt/memdisk# ./hide_change DST/linux-5.9.8/README 10 terre:/mnt/memdisk# diff -rq SRC DST Files SRC/linux-5.9.8/README and DST/linux-5.9.8/README differ terre:/mnt/memdisk# rsync -arvHAXS SRC/* DST sending incremental file list sent 1,254,054 bytes received 5,215 bytes 839,512.67 bytes/sec total size is 954,980,692 speedup is 758.36 terre:/mnt/memdisk# diff -rq SRC DST <e>Files SRC/linux-5.9.8/README and DST/linux-5.9.8/README differ terre:/mnt/memdisk#`

modifying the backup (the directory we sync with), rsync does not report any difference and the backup stay corrupted.

##### Tar

`terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# tar -czf backup.tar.gz SRC terre:/mnt/memdisk# ls -l backup* -rw-r--r-- 1 root root 183659664 Nov 17 18:11 backup.tar.gz terre:/mnt/memdisk# ./hide_change backup.tar.gz 1 terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xzf ../backup.tar.gz gzip: stdin: not in gzip format tar: Child returned status 1 tar: Error is not recoverable: exiting now terre:/mnt/memdisk/DST# terre:/mnt/memdisk/DST# find . -ls 1720964 0 drwxr-xr-x 2 root root 40 Nov 17 18:56 . terre:/mnt/memdisk/DST#`

Modifying the first byte leads to a completely unusable backup. Nothing got restored at all. Let's see what going on when modifying a single bit in the middle of the backup:

`terre:/mnt/memdisk# ls -l backup* -rw-r--r-- 1 root root 183659664 Nov 17 18:11 backup.tar.gz terre:/mnt/memdisk# ./hide_change backup.tar.gz 734638656 terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xf ../backup.tar.gz tar: Skipping to next header gzip: stdin: invalid compressed data--crc error gzip: stdin: invalid compressed data--length error tar: Child returned status 1 <e>tar: Error is not recoverable: exiting now terre:/mnt/memdisk/DST# terre:/mnt/memdisk/DST# diff -rq ../SRC SRC | wc -l diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/arc: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/arm: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/arm64: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/c6x: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/h8300: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/microblaze: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/mips: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/nios2: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/openrisc: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/powerpc: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/sh: No such file or directory diff: SRC/linux-5.9.8/scripts/dtc/include-prefixes/xtensa: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/copyloops/copy_mc_64.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/copyloops/copyuser_64.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/copyloops/copyuser_power7.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/copyloops/memcpy_64.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/copyloops/memcpy_power7.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/nx-gzip/include/vas-api.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/primitives/asm/asm-compat.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/primitives/asm/asm-const.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/primitives/asm/feature-fixups.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/primitives/asm/ppc_asm.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/primitives/word-at-a-time.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/stringloops/memcmp_32.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/stringloops/memcmp_64.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/stringloops/strlen_32.S: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/vphn/asm/lppaca.h: No such file or directory diff: SRC/linux-5.9.8/tools/testing/selftests/powerpc/vphn/vphn.c: No such file or directory <e>150 terre:/mnt/memdisk/DST# find ../SRC | wc -l 74726 terre:/mnt/memdisk/DST# find SRC | wc -l 32615 terre:/mnt/memdisk/DST#`

Only 32615 files on the 74726 that were saved could be restored. Assuming the problem is due to the fact the backup is compressed, let's see tar without compression:

`terre:/mnt/memdisk# tar -cf backup.tar SRC terre:/mnt/memdisk# ls -l backup* -rw-r--r-- 1 root root 1011312640 Nov 17 19:28 backup.tar terre:/mnt/memdisk# ./hide_change backup.tar 1 terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xf ../backup.tar tar: This does not look like a tar archive <e>tar: Skipping to next header tar: Exiting with failure status due to previous errors terre:/mnt/memdisk/DST# diff -rq ../SRC SRC terre:/mnt/memdisk/DST# echo $? 0 terre:/mnt/memdisk/DST#`

Without compression, a tar backup is much more reliable, however we now need more than 5 times storage space to hold the backup. Let's see what happens when we modify a single bit in the midle of the backup:

`terre:/mnt/memdisk# ./hide_change backup.tar 4045250560 terre:/mnt/memdisk# rm -rf DST terre:/mnt/memdisk# mkdir DST terre:/mnt/memdisk# cd DST terre:/mnt/memdisk/DST# tar -xf ../backup.tar terre:/mnt/memdisk/DST# terre:/mnt/memdisk/DST# diff -rq ../SRC SRC Files ../SRC/linux-5.9.8/drivers/media/pci/bt8xx/bttv-cards.c and SRC/linux-5.9.8/drivers/media/pci/bt8xx/bttv-cards.c differ terre:/mnt/memdisk/DST#`

the backup restoration suceeded according to tar but the corruption has been completely ignored!!! The result is both a corrupted backup and a corrupted restored data, with no notification at all...

##### Parchive

We can increase the robustness of any file or set of files by mean of Parchive software. If its use is adapted to tar and dar it is not adapted to rsync due to the directory tree structure it uses for its backup. We will thus here measure the par2create (Parchive) execution time compared to backup time of tar and dar.

`devuan:/mnt/memdisk# mkdir SRC devuan:/mnt/memdisk# cp --preserve -r /usr SRC devuan:/mnt/memdisk# time tar -czf backup.tar.gz SRC 62.550u 3.148s <e>1:01.75 106.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -c backup -z6 -1 0 -at -R SRC -q 60.287u 1.152s <e>1:01.45 99.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# ls -l backup.* -rw-r--r-- 1 root root <e>601976990 Dec 1 10:48 backup.1.dar -rw-r--r-- 1 root root <e>588260243 Dec 1 10:47 backup.tar.gz devuan:/mnt/memdisk# time par2create -r5 -n1 -q backup.tar.gz Opening: backup.tar.gz Done 94.465u 0.535s <e>0:05.74 1654.8% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time par2create -r5 -n1 -q backup.1.dar Opening: backup.1.dar Done 110.048u 0.364s <e>0:06.19 1783.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

We see that the redundancy process is not negligible: for 5% of data redundancy, we need around 10% of extra CPU time. Things get worse when the size of the data to process by Parchive increases and when the disk I/O comes to play (here the /mnt/memdisk was an in-memory tmpfs filesystem). This is the case when the amount of data to backup is larger than the available RAM space, which is a quite frequent situation:

`devuan:~/tmp# du -sh SRC <e>27G SRC devuan:~/tmp# free -m . total used free shared buff/cache available Mem: <e>15776 560 433 1 14782 14823 Swap: 0 0 0 devuan:~/tmp#`

We are now placed in this context, having a 27 GiB data to backup on a machine having only around 16 GiB of RAM.

`devuan:~/tmp# time dar -c backup -z6 -at -1 0 -R SRC -q -E 'par2create -r5 -n1 -q %b.%N.%e' Opening: backup.1.dar Done 15237.777u 105.379s <e>36:48.53 694.7% 0+0k 158854544+57572120io 158pf+0w devuan:~/tmp#`

This execution time of around 36 mn above can be improved by using multiple slices. Choosing a slice size smaller than the available RAM let Parchive compute parity data right after each slice has been generated, while it is still in the disk cache (RAM), bypassing the corresponding disks I/O we had previously in a second time:

`devuan:~/tmp# time dar -c backup_splitted -z6 -at -1 0 -R SRC -q -E 'par2create -r5 -n1 -q %b.%N.%e' <e>-s 1G Opening: backup_splitted.1.dar Done Opening: backup_splitted.2.dar Done [...] Opening: backup_splitted.26.dar Done Opening: backup_splitted.27.dar Done 6040.106u 49.201s <e>21:58.73 461.7% 0+0k 61862640+57567104io 123pf+0w devuan:~/tmp#`

The total execution time dropped to around 22 mn! Which makes a 40% time reduction. By the way, having here 27 files of 1 GiB is also easier to manipulate (file transfer, copy to removable media,...) than a huge equivalent file of 27 GiB.

`devuan:~/tmp# time tar -czf backup.tar.gz SRC 837.662u 80.776s <e>18:26.08 83.0% 0+0k 54926128+54799696io 6pf+0w devuan:~/tmp# time par2create -r5 -n1 -q backup.tar.gz Opening: backup.tar.gz Done 13352.393u 71.390s <e>18:24.34 1215.5% 0+0k 95000064+2772144io 9pf+0w devuan:~/tmp#`

We get the same execution time (18 mn) for both tar and par2 for thus a total of 36 mn. This same time for both software while the real CPU usage is much more important for par2, clearly shows that the slowest operation was the disk I/O. Else the overall time of the operation is similar to what we say with dar above, except that we cannot use multi-volume to speed up the operation as we did with dar: tar is not able to compress *and* produce multi-volume backup: What we would gain on one side would be lost on the other side...

### Execution Performance

In order to compare performance in a fair manner, we have to take into consideration that some CPU intensive features are not implement by all softwares or have different default values:

- The default compression level differs: dar uses level 9 by default while rsync and tar use level 6 which is faster. Only dar and rsync seems able let the user set this value, so we will have to manually set dar to use level 6 too: `-z6 option`
- the disk usage optimization is not supported by tar so we will not activate sparse file detection and optimization (for rsync no `-S option`) and disable it for dar (activated by default): `-1 0 option`
- dar spends non negligible CPU cycle to duplicate metadata along the backup, this is one of the root of exclusive robusness brought by dar backups, but this is an optional feature. We will disable it using the `-at option`
- rsync and dar calculate checksum on the saved data, while tar completely skips this data protection. Unfortunately this is not possible to disable for both, thus tar will have a speed advantage thanks to this difference.

For the data reduction, we will first compare with the same feature set, then activate all specific features each software can leverage to improve data reduction, and will measure the execution time impact.

All performance aspects are not interesting in all use cases. We can distinguish two main types of use cases:

- A first set of tests will cover the copy operations, they will not use any compression, focusing only on execution time.
- A second set of tests will cover the backup operations, use compression and focus on the data reduction mainly but also on the execution time.

When using rsync as a backup tool (at the opposite of a copy operation), we assume the remote (or local) copy is the backup, and thus restoring implies syncing back this remote (or local) copy to the place the original data was located and has been lost.

To prepare the data under test we used:

- a big ISO file
- a full linux system

All data to backup or to copy has been stored in a tmpfs, which is also the destination of the created backups and restored data. The swap has been disabled to avoid any disk I/O penalty, in the intention to provide a fair comparison environment (avoiding disk cache variable performance).

`devuan:/mnt/memdisk# free total used free shared buff/cache available Mem: 16155172 691764 151608 8780152 15311800 6299316 <e>Swap: 0 0 0 devuan:/mnt/memdisk# df -h . Filesystem Size Used Avail Use% Mounted on <e>tmpfs 14G 4.0K 14G 1% /mnt/memdisk devuan:/mnt/memdisk#`

To prepare the Linux system under backup we installed the Devuan system a few days before in a VM. On day D, a full backup has been executed, we updated/upgraded the system using the disto package manager and we made a differential backup based on the first one, both backup being wrote to the testing machine (bare-metal server) that was used for the performance tests:

`root@Georges:~# dar -c sftp://denis@10.13.30.163/home/denis/tmp/full -zlz4 -R / -M -D --hash sha1 -afile-auth -C cat_full root@Georges:~# apt-get update [...] root@Georges:~# apt-get upgrade [...] root@Georges:~# dar -c sftp://denis@10.13.30.163/home/denis/tmp/diff -A cat_full -zlz4 -R / -M -D --hash sha1 -afile-auth root@Georges:~#`

Back on the testing host (the bare-metal server at 10.13.30.163) we restored the data for the performance test, the following way: excluding FSA and EA to avoid a tone of warning as those are not supported on tmpfs filesystem:

`devuan:/mnt/memdisk# mkdir state-1 devuan:/mnt/memdisk# mkdir state-2 devuan:/mnt/memdisk# dar -x ~denis/tmp/full -R state-1 --fsa-scope none -u "*" -------------------------------------------- 136836 inode(s) restored including 27 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 136836 -------------------------------------------- EA restored for 3 inode(s) FSA restored for 0 inode(s) -------------------------------------------- devuan:/mnt/memdisk# dar -x ~denis/tmp/full -R state-2 --fsa-scope none -u "*" -------------------------------------------- 136836 inode(s) restored including 27 hard link(s) 0 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 136836 -------------------------------------------- EA restored for 3 inode(s) FSA restored for 0 inode(s) -------------------------------------------- devuan:/mnt/memdisk# dar -x ~denis/tmp/diff -R state-2 --fsa-scope none -u "*" -w -------------------------------------------- 568 inode(s) restored including 0 hard link(s) 136670 inode(s) not restored (not saved in archive) 0 inode(s) not restored (overwriting policy decision) 0 inode(s) ignored (excluded by filters) 0 inode(s) failed to restore (filesystem error) 0 inode(s) deleted -------------------------------------------- Total number of inode(s) considered: 137238 -------------------------------------------- EA restored for 0 inode(s) FSA restored for 0 inode(s) -------------------------------------------- devuan:/mnt/memdisk#`

This leads us to have two directories `state-1` and `state-2` corresponding to the state of the Devuan machine has two days ago and today respectively.

#### Performance of copy operations

To perform the copy operation, we have decomposed the operations to precisely measure the execution time. We could have decided to pipe the backup to a second instance of the backup tool restoring the data (tar and dar only would benefit from this). But the time measurement was less easy to obtain and doing that way does not seem to provide any noticable speed improvement. The data used here in `SRC1` is a single big ISO file (a Devuan installation DVD image).

##### Dar

`devuan:/mnt/memdisk# time dar -c copy -1 0 -at -R SRC1 -q 1.834u 2.909s <e>0:04.87 97.1% 0+0k 744+0io 12pf+0w devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x copy -R DST -q 1.563u 2.836s <e>0:04.41 99.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# du -sh SRC1 DST 4.4G SRC1 4.4G DST devuan:/mnt/memdisk# diff -r SRC1 DST devuan:/mnt/memdisk# echo $? 0 devuan:/mnt/memdisk# rm -rf DST copy.1.dar devuan:/mnt/memdisk# time dar -c copy -1 0 -at -R SRC2 -q 4.739u 3.683s <e>0:08.44 99.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x copy -R DST -q 4.449u 3.872s <e>0:08.34 99.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# du -sh SRC2 DST 4.1G SRC2 4.1G DST devuan:/mnt/memdisk# find DST | wc -l 136837 devuan:/mnt/memdisk#`

The overall copy time for dar is:

- 9.18 seconds for the big file in `SRC1`
- 16.78 seconds for the full linux system in `SRC2`

##### Rsync

`devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time rsync -arH SRC1 DST 23.088u 5.420s <e>0:15.28 186.5% 0+0k 168+0io 4pf+0w devuan:/mnt/memdisk# diff -r SRC1/ DST/SRC1/ devuan:/mnt/memdisk# echo $? 0 devuan:/mnt/memdisk# rm -rf DST devuan:/mnt/memdisk# time rsync -arH SRC2 DST 22.408u 8.560s <e>0:16.59 186.6% 0+0k 1224+0io 6pf+0w devuan:/mnt/memdisk# du -sh SRC2 DST 4.1G SRC2 4.1G DST devuan:/mnt/memdisk# find DST | wc -l 136838 devuan:/mnt/memdisk#`

The overall copy time for rsync is:

- 15.28 seconds for the big file in `SRC1`
- 16.59 seconds for the full linux system in `SRC2`

##### Tar

`devuan:/mnt/memdisk# cd SRC1 devuan:/mnt/memdisk/SRC1# time tar -cf ../copy.tar * 0.343u 2.756s <e>0:03.10 99.6% 0+0k 104+0io 1pf+0w devuan:/mnt/memdisk/SRC1# cd ../ devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xf ../copy.tar 0.339u 3.071s <e>0:03.41 99.7% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# cd .. devuan:/mnt/memdisk# diff -r SRC1/ DST/ devuan:/mnt/memdisk# echo $? 0 devuan:/mnt/memdisk# rm -rf DST copy.tar devuan:/mnt/memdisk# cd SRC2 devuan:/mnt/memdisk/SRC2# time tar -cf ../copy.tar * tar: tmp/.ICE-unix/19789: socket ignored tar: tmp/.X11-unix/X0: socket ignored 0.760u 2.887s <e>0:03.66 99.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC2# cd .. devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xf ../copy.tar 0.814u 3.556s <e>0:04.38 99.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# cd .. devuan:/mnt/memdisk# du -sh SRC2 DST 4.1G SRC2 4.1G DST devuan:/mnt/memdisk# find DST | wc -l 136834 devuan:/mnt/memdisk#`

the overall copy time for tar is:

- 6.51 seconds for the big file in `SRC1`
- 8.04 seconds for the full Linux system in `SRC2`

##### Cp

`devuan:/mnt/memdisk# time cp --preserve -r SRC1 DST 0.051u 2.514s <e>0:02.58 99.2% 0+0k 8+0io 1pf+0w devuan:/mnt/memdisk# diff -r SRC1 DST devuan:/mnt/memdisk# echo $? 0 devuan:/mnt/memdisk# rm -rf DST devuan:/mnt/memdisk# time cp --preserve -r SRC2 DST 0.910u 4.194s <e>0:05.15 99.0% 0+0k 288+0io 1pf+0w devuan:/mnt/memdisk# du -sh SRC2 DST <e>4.1G SRC2 4.2G DST devuan:/mnt/memdisk# find DST | wc -l 136838 devuan:/mnt/memdisk# find DST/SRC2/tmp/ -ls 2315983 0 drwxrwxrwt 4 root root 100 Dec 3 10:32 DST/SRC2/tmp/ 2315987 0 drwxrwxrwt 2 root root 60 Dec 3 10:27 DST/SRC2/tmp/.ICE-unix 2315988 0 srwxrwxrwx 1 denis denis 0 Dec 3 10:27 DST/SRC2/tmp/.ICE-unix/19789 2315985 0 drwxrwxrwt 2 root root 60 Dec 3 10:32 DST/SRC2/tmp/.X11-unix 2315986 0 srwxrwxrwx 1 root root 0 Dec 3 10:32 DST/SRC2/tmp/.X11-unix/X0 2315984 4 -r--r--r-- 1 root root 11 Dec 3 10:32 DST/SRC2/tmp/.X0-lock devuan:/mnt/memdisk#`

The overall copy time for cp is:

- 2.58 seconds for the big file in `SRC1`
- 5.15 seconds for the full Linux system in `SRC2`

cp is always the fastest and does reject the unix sockets as tar does. However it requires slightly more storage than all other softwares tested here. And if metadata (ACL, Extended Attributes, filesystem specific attributes, ...) need to be copied with data, it does not match the need.

#### Performance of Backup operations

For reference:

`devuan:/mnt/memdisk# du -sb state-* <e>4095931349 state-1 <e>4136318367 state-2 devuan:/mnt/memdisk# find state-1 | wc -l 136837 devuan:/mnt/memdisk# find state-2 | wc -l 137239 devuan:/mnt/memdisk#`

##### Dar

###### Minimal features

`devuan:/mnt/memdisk# time dar -c dar-full -R state-1 -at -1 0 -z6 -q 145.970u 3.263s <e>2:29.73 99.6% 0+0k 12344+0io 71pf+0w devuan:/mnt/memdisk# time dar -c dar-diff -R state-2 -A dar-full -at -1 0 -z6 -q -asecu 8.957u 0.959s <e>0:09.93 99.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root <e>49498524 Dec 3 16:17 dar-diff.1.dar -rw-r--r-- 1 root root <e>1580562224 Dec 3 16:16 dar-full.1.dar devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x dar-full -R DST -q 18.677u 4.244s <e>0:22.94 99.8% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -x dar-diff -R DST -q -w 2.585u 0.780s <e>0:03.48 96.5% 0+0k 1856+0io 20pf+0w devuan:/mnt/memdisk# devuan:/mnt/memdisk# time dar -x dar-full -R DST -q -w -g etc/fstab 0.934u 0.036s <e>0:00.98 97.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

for dar with minimal features (metadata no redundancy `-at`, no sparse file consideration `-1 0`):

- data reduction on storage for the full backup is: 61.41%
- data reduction on storage for the diff backup is: 98.80%
- data reduction over the network is the same as on storage
- execution time to restore a single file is: 0.98 s
- execution time to restore the full backup: 22.94 s
- execution time ot restore the diff backup: 3.48 s
- full backup time: 149.73 s
- diff backup time: 9.93 s

###### sparse file

`devuan:/mnt/memdisk# rm -rf DST *.dar devuan:/mnt/memdisk# time dar -c dar-full -R state-1 -at -z6 -q 154.971u 3.000s <e>2:37.99 99.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -c dar-diff -R state-2 -A dar-full -at -z6 -q -asecu 9.488u 0.871s <e>0:10.37 99.8% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root <e>49505251 Dec 3 16:27 dar-diff.1.dar -rw-r--r-- 1 root root <e>1578428790 Dec 3 16:25 dar-full.1.dar devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x dar-full -R DST -q 24.231u 6.110s <e>0:30.36 99.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -x dar-diff -R DST -q -w 2.677u 0.793s <e>0:03.48 99.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -x dar-full -R DST -q -w -g etc/fstab 1.067u 0.053s <e>0:01.13 98.2% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

for dar with default features (metadata no redundancy `-at`, with sparse file consideration (activated by default))

- data reduction on storage for the full backup is: 61.46%
- data reduction on storage for the diff backup is: 98.80%
- data reduction over the network is the same as on storage
- execution time to restore a single file is: 1.13 s
- execution time to restore the full backup: 30.36 s
- execution time ot restore the diff backup: 3.48 s
- full backup time: 157.99 s
- diff backup time: 10.37 s

###### Sparse file and binary delta

`devuan:/mnt/memdisk# rm -rf DST *.dar devuan:/mnt/memdisk# time dar -c dar-full -R state-1 -at -z6 --delta sig -q 159.262u 3.332s <e>2:42.62 99.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -c dar-diff -R state-2 -A dar-full -at -z6 -q -asecu 6.149u 0.950s <e>0:07.11 99.7% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# ls -l *.dar -rw-r--r-- 1 root root <e>23883368 Dec 3 16:39 dar-diff.1.dar -rw-r--r-- 1 root root <e>1602481058 Dec 3 16:38 dar-full.1.dar devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x dar-full -R DST -q 24.169u 6.163s <e>0:30.35 99.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -x dar-diff -R DST -q -w 2.481u 0.942s <e>0:03.44 99.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time dar -x dar-full -R DST -q -w -g etc/fstab 1.205u 0.059s <e>0:01.27 98.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

for dar with advanced features (metadata no redundancy `-at`, with binary delta computation `--delta sig`):

- data reduction on storage for the full backup is: 60,87%
- data reduction on storage for the diff backup is: 99.42%
- data reduction over the network is the same as on storage
- execution time to restore a single file is: 1.27 s
- execution time to restore the full backup: 30.35 s
- execution time ot restore the diff backup: 1.27 s
- full backup time: 162.62 s
- diff backup time: 7.11 s

##### Rsync

###### minimal features

`devuan:/mnt/memdisk# mkdir rsync-backup devuan:/mnt/memdisk# time rsync -arHz --info=stats state-1/* rsync-backup sent <e>1,585,540,014 bytes received 2,174,472 bytes 10,080,726.90 bytes/sec total size is 4,260,538,564 speedup is 2.68 202.640u 8.503s <e>2:36.98 134.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# devuan:/mnt/memdisk# time rsync -arHz --info=stats --no-whole-file state-2/* rsync-backup <e>sent 29,077,377 bytes received 216,581 bytes 3,446,348.00 bytes/sec total size is 4,300,916,222 speedup is 146.82 7.555u 1.115s <e>0:07.33 118.1% 0+0k 1784+0io 7pf+0w devuan:/mnt/memdisk# du -sb rsync-backup <e>4136318307 rsync-backup devuan:/mnt/memdisk# devuan:/mnt/memdisk# rm -rf state-1 devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time rsync -arHz --info=stats --no-whole-file rsync-backup/* DST <e>sent 1,599,585,756 bytes received 2,181,147 bytes 10,105,784.88 bytes/sec total size is 4,300,916,222 speedup is 2.69 204.192u 8.306s <e>2:37.81 134.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time rsync -arHz --info=stats rsync-backup/etc/fstab DST/etc sent 44 bytes received 12 bytes 112.00 bytes/sec total size is 664 speedup is 11.86 <e>0.001u 0.002s 0:00.00 0.0% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

for rsync with minimal features (no sparse file consideration):

- data reduction on storage for the full backup is: 0%
- data reduction on storage for the diff backup is: 0%
- data reduction over the network for the full backup is: 61.23%
- data reduction over the network for the diff backup is: 99.29%
- execution time to restore a single file is: 0.003 s
- execution time ot restore the full+diff backup: 157.81 s (this is not due to the `--no-whole-file` see next text)
- full backup time: 156.98 s
- diff backup time: 7.33 s

###### sparse file + binary delta

`devuan:/mnt/memdisk# mkdir rsync-backup devuan:/mnt/memdisk# time rsync -arHSz --info=stats state-1/* rsync-backup <e>sent 1,585,540,014 bytes received 2,174,460 bytes 8,605,498.50 bytes/sec total size is 4,260,538,564 speedup is 2.68 232.038u 13.137s <e>3:03.44 133.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# time rsync -arHSz --info=stats --no-whole-file state-2/* rsync-backup <e>sent 29,077,381 bytes received 216,577 bytes 3,446,348.00 bytes/sec total size is 4,300,916,222 speedup is 146.82 7.305u 1.275s <e>0:07.04 121.7% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# du -sb rsync-backup <e>4136318307 rsync-backup devuan:/mnt/memdisk# rm -rf state-1 devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time rsync -arHSz --info=stats rsync-backup/* DST sent 1,599,585,756 bytes received 2,181,219 bytes 10,042,426.18 bytes/sec total size is 4,300,916,222 speedup is 2.69 205.089u 12.354s <e>2:38.39 137.2% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# devuan:/mnt/memdisk# time rsync -arHSz --info=stats rsync-backup/etc/fstab DST/etc sent 44 bytes received 12 bytes 112.00 bytes/sec total size is 664 speedup is 11.86 0.001u 0.002s 0:00.00 0.0% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

for rsync with advanced features (sparse file consideration `-S option`, binary delta `--no-whole-file`)

- data reduction on storage for the full backup is: 0%
- data reduction on storage for the diff backup is: 0%
- data reduction over the network for the full backup is: 60.64%
- data reduction over the network for the diff backup is: 99.28%
- execution time to restore a single file is: 0.003 s
- execution time to restore the full+diff backup: 158.39 s
- full backup time: 183.44 s
- diff backup time: 7.04 s

##### Tar

###### minimal features

`devuan:/mnt/memdisk# cd state-1 devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czf ../tar-full.tar.gz * tar: tmp/.ICE-unix/19789: socket ignored tar: tmp/.X11-unix/X0: socket ignored 153.624u 8.676s 2:31.71 106.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czf ../tar-diff.tar.gz * 0.809u 0.369s 0:00.98 118.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/state-1# cd .. devuan:/mnt/memdisk# ls -l tar* -rw-r--r-- 1 root root 765425 Dec 3 16:49 tar-diff.tar.gz -rw-r--r-- 1 root root 1546464033 Dec 3 16:48 tar-full.tar.gz devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xzf ../tar-full.tar.gz 27.106u 6.756s 0:26.72 126.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# cd .. devuan:/mnt/memdisk# diff --no-dereference -r state-1 DST Only in state-1: .cache Only in state-1/tmp/.ICE-unix: 19789 Only in state-1/tmp/.X11-unix: X0 devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xzf ../tar-diff.tar.gz 0.183u 0.085s 0:00.18 144.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST#`

Doing that way, the tar differential backup is empty: it only contains empty directories, no file data. We will apply the changes over `state-1` rather than already setup changes at `state-2`. This seems to mean that if the system clock is wrong or was wrong at the time a file was modified (like daylight saving? or before NTP synchronization at system startup), which is the same as here, were the changes have been brought before full backup was done, those changes will not be backed up by tar, while the file's attributes (file size, last modification date,...) changed.

`devuan:/mnt/memdisk# cd state-1 devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czf ../tar-full.tar.gz * tar: tmp/.ICE-unix/19789: socket ignored tar: tmp/.X11-unix/X0: socket ignored 150.751u 8.299s <e>2:28.59 107.0% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/state-1# cd .. devuan:/mnt/memdisk# dar -x ~denis/tmp/diff -R state-1 --fsa-scope none -u "*" -w -q devuan:/mnt/memdisk# cd state-1 devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czf ../tar-diff.tar.gz * 6.147u 0.559s <e>0:06.40 104.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/state-1# cd .. devuan:/mnt/memdisk# ls -l tar* snapshot.file -rw-r--r-- 1 root root <e>3350869 Dec 3 17:08 snapshot.file -rw-r--r-- 1 root root <e>44607904 Dec 3 17:08 tar-diff.tar.gz -rw-r--r-- 1 root root <e>1546448179 Dec 3 17:04 tar-full.tar.gz devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd dst dst: No such file or directory. devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xzf ../tar-full.tar.gz 26.807u 7.020s <e>0:26.72 126.5% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# time tar -xzf ../tar-diff.tar.gz 1.492u 0.381s <e>0:01.48 126.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# time tar -xzf ../tar-full.tar.gz etc/fstab 25.219u 2.581s <e>0:25.15 110.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST#`

for tar with minimal features (no sparse file consideration):

- data reduction on storage for the full backup is: 62.16%
- data reduction on storage for the diff backup is: 98.92%
- data reduction over the network is the same as on storage
- execution time to restore a single file is: 25.15 s
- execution time to restore the full backup: 26.72 s
- execution time ot restore the diff backup: 1.48 s
- full backup time: 148.59 s
- diff backup time: 6.40 s

###### sparse file

Whe had to recreate `state-1` as we needed modifying it at previous test

`devuan:/mnt/memdisk# rm tar* snapshot.file devuan:/mnt/memdisk# cd state-1 devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czSf ../tar-full.tar.gz * tar: tmp/.ICE-unix/19789: socket ignored tar: tmp/.X11-unix/X0: socket ignored 152.878u 10.155s <e>2:29.38 109.1% 0+0k 1520+0io 18pf+0w devuan:/mnt/memdisk/state-1# dar -x ~denis/tmp/diff --fsa-scope none -u "*" -w -q devuan:/mnt/memdisk/state-1# time tar --listed-incremental=../snapshot.file -czSf ../tar-diff.tar.gz * 6.369u 0.752s <e>0:06.55 108.5% 0+0k 3992+0io 16pf+0w devuan:/mnt/memdisk/state-1# cd .. devuan:/mnt/memdisk# ls -l tar* snap* -rw-r--r-- 1 root root <e>3350870 Dec 3 17:29 snapshot.file -rw-r--r-- 1 root root <e>44604194 Dec 3 17:29 tar-diff.tar.gz -rw-r--r-- 1 root root <e>1546226992 Dec 3 17:27 tar-full.tar.gz devuan:/mnt/memdisk# rm -rf DST devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time tar -xzSf ../tar-full.tar.gz 27.331u 7.774s <e>0:26.27 133.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# time tar -xzSf ../tar-diff.tar.gz 1.547u 0.487s <e>0:01.50 134.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# time tar -xzSf ../tar-full.tar.gz etc/fstab 25.068u 2.565s <e>0:25.00 110.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST#`

for tar with advanced features (sparse file consideration `-S option`):

- data reduction on storage for the full backup is: 62.16%
- data reduction on storage for the diff backup is: 98.92%
- data reduction over the network is the same as on storage
- execution time to restore a single file is: 25.0 s
- execution time to restore the full backup: 26.27 s
- execution time ot restore the diff backup: 1.50 s
- full backup time: 149.38 s
- diff backup time: 6.55 s

#### Ciphering performance

We evaluate here ciphering and deciphering performance. To compare on the same base we use the following parameters:

- AES-256 algorithm with CBC mode
- pkcs5 v2 (pbkdf2) key derivation function (KDF) algorithm
- KDF with 100,000 iterations
- salt
- password provided on command-line (insecure) to not depend on user or disk access
- local system to backup and backup repository on a tmpfs filesystem
- swap has been disabled to avoid tmpfs latency in case it would have been swapped out

To measure the ciphering time only, we will not use compression, though most of the time compression should be used due to the use case encryption matches: relatively long time storage and/or costing cloud space and network transfer time or limited removable media storage.

The content that will be backed up is a copy of `/usr` directory tree. We will measure:

- the time to backup
- the time to restore the whole backup
- and the time to restore just the "diff" binary

`devuan:/mnt/memdisk# mkdir SRC devuan:/mnt/memdisk# cp --preserve -r /usr SRC devuan:/mnt/memdisk# time dar -c backup -K "aes256:hello world!" --kdf-param 100000:sha1 -R SRC -q -at -1 0 9.213u 3.245s <e>0:05.38 231.4% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# time dar -x backup -K "hello world!" -R DST -q Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted 4.481u 2.628s <e>0:03.75 189.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# rm -rf DST/usr devuan:/mnt/memdisk# time dar -x backup -K "hello world!" -R DST -q -g usr/bin/diff Warning, the archive backup has been encrypted. A wrong key is not possible to detect, it would cause DAR to report the archive as corrupted 0.419u 0.025s <e>0:00.42 102.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk#`

For dar the operation took:

- 5.38 seconds to backup with encryption
- 3.75 seconds to restore the whole ciphered backup
- 0.42 seconds to restore a single file from the ciphered backup

dar is twice quicker to uncipher than to cipher the whole archive, but restoring a particular file is quite immediate. By default, dar uses argon2 for KDF, which is the most secure algorithm as of year 2020 to derive a key, but we had to adapt to openssl used with tar that does not (yet) support this algorithm.

To avoid plain-text attack a variable length elastic buffer containing random data is encrypted with the rest of the backed up files at the beginning and at the end of the backup, this has some performance penalties (time to generate and time to cipher/decipher). This explains why two identical invocations of dar produce backups of different sizes and execution times:

`devuan:/mnt/memdisk# time dar -c backup -K "aes256:hello world!" -at -1 0 -R SRC -q -w 9.782u 3.413s <e>0:06.28 210.0% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# la backup.1.dar -rw-r--r-- 1 root root 1572<e>706497 Nov 9 14:50 backup.1.dar devuan:/mnt/memdisk# time dar -c backup -K "aes256:hello world!" -at -1 0 -R SRC -q -w 9.173u 2.845s <e>0:05.50 218.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk# la backup.1.dar -rw-r--r-- 1 root root 1572<e>655217 Nov 9 14:50 backup.1.dar devuan:/mnt/memdisk#`

rsync has no way to store the backup ciphered. Testing directly tar now:

tar has not support for ciphering. Though it seems the some use openssl workaround this restriction. To measure the execution time we have to create as script that pipes tar and openssl so we can measure the execution time of this script as a whole. There is thus one script for backup and one for the restoration of tar+openssl.

`devuan:/mnt/memdisk# cat > tar.backup #!/bin/bash if [ -z "$1" ] ; then echo "usage: $0 <backup name> [ <file or dir> ]" exit 1 fi tar -cf - "$2" | openssl enc -e -aes256 -out "$1" -pbkdf2 -iter 100000 -salt -pass pass:"hello world!" devuan:/mnt/memdisk# devuan:/mnt/memdisk# chmod u+x tar.backup devuan:/mnt/memdisk# cd SRC devuan:/mnt/memdisk/SRC# time ../tar.backup ../backup.tar.crypted usr 3.954u 2.498s <e>0:04.69 137.3% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC# cd .. devuan:/mnt/memdisk# ls -l backup.tar.crypted -rw-r--r-- 1 root root 1603594272 Nov 9 13:33 backup.tar.crypted devuan:/mnt/memdisk# devuan:/mnt/memdisk# cat > tar.restore #!/bin/bash if [ -z "$1" ] ; then echo "usage: $0 <tar.crypted file> [<file or dir>]" exit 1 fi openssl enc -d -aes256 -in "$1" -pbkdf2 -iter 100000 -salt -pass pass:"hello world!" | tar -x "$2" devuan:/mnt/memdisk# chmod u+x tar.restore devuan:/mnt/memdisk# rm -rf DST devuan:/mnt/memdisk# mkdir DST devuan:/mnt/memdisk# cd DST devuan:/mnt/memdisk/DST# time ../tar.restore ../backup.tar.crypted 1.807u 2.821s <e>0:02.70 171.1% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# devuan:/mnt/memdisk/DST# rm -rf usr devuan:/mnt/memdisk/DST# time ../tar.restore ../backup.tar.crypted usr/bin/diff 1.336u 1.428s <e>0:01.79 153.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/DST# find . ./usr ./usr/bin ./usr/bin/diff devuan:/mnt/memdisk/DST#`

For tar the operation took:

- 4.69 seconds for backup
- 2.70 seconds to restore the whole backup
- 1.79 seconds to restore a single file

tar as dar is also twice longer to cipher than to decipher, this seems to be related to the algorithm itself. Though tar is a bit faster than dar but lacks protection against clear-text: the generated encrypted backup have the exact same sizes at one byte precision, this means the blocks boundaries and tar file internal structure always lay at the same file offset for a given content:

`devuan:/mnt/memdisk/SRC# time ../tar.backup ../backup.tar.crypted usr 4.112u 2.343s 0:04.72 136.6% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC# ls -l ../bac backup.1.dar backup.tar.crypted devuan:/mnt/memdisk/SRC# ls -l ../backup.tar.crypted -rw-r--r-- 1 root root <e>1603594272 Nov 9 14:56 ../backup.tar.crypted devuan:/mnt/memdisk/SRC# time ../tar.backup ../backup.tar.crypted usr 3.952u 2.564s 0:04.79 135.9% 0+0k 0+0io 0pf+0w devuan:/mnt/memdisk/SRC# ls -l ../backup.tar.crypted -rw-r--r-- 1 root root <e>1603594272 Nov 9 14:56 ../backup.tar.crypted devuan:/mnt/memdisk/SRC#`

## Scripts used in this benchmark

The following scripts are also available for download from the directory

### historization_feature script

`#!/bin/bash if [ -z "$1" -o -z "$2" ] ; then echo "usage: $0 <dir> {phase1 | phase2}" exit 1 fi dir="$1" phase="$2" case "$phase" in phase1) if [ -e "$dir" ] ; then echo "$dir exists, remove it first" exit 2 fi mkdir "$dir" echo "Hello World!" > "$dir/A.txt" echo "Bonjour tout le monde !" > "$dir/B.txt" ;; phase2) if [ ! -d "$dir" ] ; then echo "$dir does not exist or is not a directory, run phase1 first" exit 2 fi rm -f "$dir/A.txt" echo "Buongiorno a tutti !" > "$dir/C.txt" ;; *) echo "unknown phase" exit 2 ;; esac`

### always_change script

`#!/bin/bash if [ -z "$1" ] ; then echo "usage: $0 <filename>" exit 1 fi while /bin/true ; do touch "$1" ; done`

### bitflip script

`#!/bin/bash if [ -z "$1" -o -z "$2" ] ; then echo "usage: $0 <offset in bit> <file>" echo "flip the bit of the file located at the provided offset" exit 1 fi offbit=$1 file="$2" offbyte=$(( $offbit / 8 )) bitinbyte=$(( $offbit - ($offbyte * 8) )) readbyte=`xxd -s $offbyte -p -l 1 "$file"` mask=$(( 1 << $bitinbyte )) newbyte=$(( 0x$readbyte ^ $mask )) hexanewbyte=`printf "%.2x" $newbyte` echo $hexanewbyte | xxd -p -l 1 -s $offbyte -r - "$file"`

### build_test_tree.bash script

`#!/bin/bash if [ -z "$1" ] ; then echo "usage: $0 <directory>" exit 1 fi if [ -e "$1" ] ; then echo "$1 already exists, remove it or use another directory name" exit 1 fi if ! dar -V > /dev/null ; then echo "need dar to copy unix socket to the test tree" exit 1 fi mkdir "$1" cd "$1" # creating mkdir "SUB" dd if=/dev/zero of=plain_zeroed bs=1024 count=1024 dd if=/dev/urandom of=random bs=1024 count=1024 dd if=/dev/zero of=sparse_file bs=1 count=1 seek=10239999 ln -s random SUB/symlink-broken ln -s ../random SUB/symlink-valid mkfifo pipe mknod null c 3 1 mknod fd1 b 2 1 dar -c - -R / -g dev/log -N -Q -q | dar -x - --sequential-read -N -q -Q ln sparse_file SUB/hard_linked_sparse_file ln dev/log SUB/hard_linked_socket ln pipe SUB/hard_linked_pipe # modifying dates and permissions sleep 2 chown nobody random chown -h bin SUB/symlink-valid chgrp -h daemon SUB/symlink-valid sleep 2 echo hello >> random sleep 2 cat < random > /dev/null # adding Extend Attributes, assuming the filesystem as user_xattr and acl option set setfacl -m u:nobody:rwx plain_zeroed && setfattr -n "user.hello" -v "hello world!!!" plain_zeroed || (echo "FAILED TO CREATE EXTENDED ATTRIBUTES" && exit 1) # adding filesystem specific attributes chattr +dis plain_zeroed`

### hide_change script

`#!/bin/bash if [ -z "$1" ] ; then echo "usage: $0 <filename> [<bit offset>]" echo "modify one bit and hide the change" exit 1 fi atime=`stat "$1" | sed -rn -s 's/^Access:\s+(.*)\+.*/\1/p'` mtime=`stat "$1" | sed -rn -s 's/^Modify:\s+(.*)\+.*/\1/p'` bitoffset="$2" if [ -z "$bitoffset" ] ; then bitoffset=2 fi ./bitflip "$bitoffset" "$1" touch -d "$mtime" "$1" touch -a -d "$atime" "$1"`