Features

For many people hard disks provide the form of persistent storage that is most readily available and cheapest per MB. I think that rdiff-backup is often the best way to back one hard drive to another.

Easy to use: In most cases, the command
```
     rdiff-backup dir1 dir2
```
will work out-of-the-box to backup dir1 to dir2.
```
     rdiff-backup dir1 user@system::/dir2
```
will backup dir1 to dir2 on a different system (provided rdiff-backup is installed on both systems). rdiff-backup also comes with a lot of up-to-date documentation.
Creates mirror: rdiff-backup makes the backup directory into an almost exact copy of the source directory (the only difference is one extra subdirectory on the backup side). If you delete a file from the source directory you can simply copy it from the backup directory, use "find" or "locate" to find the file, or use any other familiar utility. Also, if the two directories are on different disks, you can recover almost immediately if the disk containing the source directory crashes, just by mounting the backup directory where the source directory used to be.
Keeps increments: Normally, with a mirror, any changes made to the source directory are immediately sent to the backup directory, and old changes are lost. rdiff-backup saves those changes in the form of reverse diffs, so you can recover the older form of the file.

For instance, suppose last week you deleted half of some document, thinking that what you had written was garbage. Yesterday, your backup event ran, saving these changes. Today you realize that you were on to something and want what you deleted back. If you just mirrored, you would be out of luck, since the copy on your mirror would be the newer one. With rdiff-backup, the newer version would indeed be present, but in a special directory (rdiff-backup-data/) there would be a file that recorded this change. Running rdiff-backup on this file recovers the version from a week ago.
Preserves all information: Whether you restore from the mirror directory or from an earlier incremental backup, rdiff-backup will reproduce your files exactly as they were. Files missing at the time of backup will also be missing after the restore. Files hard linked when backed up will be hard linked after the restore. rdiff-backup also preserves permissions, user and group ownership, modification time, device files, fifos, and symlinks.

Sometimes it is impossible for the information to be replicated exactly on the destination. For instance, ownership cannot usually be replicated without root access at the destination; windows file systems may not be case sensitive and have no ownership at all. rdiff-backup records file metadata in a separate file so that all information is preserved even if the destination file system is missing features.
Space efficient: Suppose you have a large database file that changes a little bit every day. A normal incremental backup would keep saving copy after copy of this database, wasting a lot of space. rdiff-backup uses librsync, which implements the same efficient diffing algorithm that rsync uses. It works on binary files as well as text, so only a fraction of the data in your database would be saved in each incremental backup.
Bandwidth efficient: rdiff-backup depends on librsync, and thus uses the same diffing algorithm as rsync (rsync and rdiff-backup strictly speaking do not share any code however). As a result, when when writing to a remote location, rdiff-backup will only send diffs over and can use much less bandwidth than, say, ftp or scp.

For instance, suppose you slightly alter large file A to make large file A', and A is still on the remote system. When rdiff-backup is run, it will only send over the diff A->A' (in order to "copy" A' to the remote system). Neither A nor A' needs to be sent in its entirety.
Transparent data format: Except for recording the hard link structure of old data sets, rdiff-backup doesn't absolutely require any data files formatted specifically for rdiff-backup. So if you want to stop using rdiff-backup in the future, you won't be stuck with any undecipherable files in some strange format. As noted above, the mirror directory will just be a copy of the source directory as it was when rdiff-backup was last run. Earlier states of your files are saved just by 1) keeping a copy of them, 2) in diff form as produced by rdiff, or 3) as a gzipped version of 1 or 2.
Filesystem feature autodetection: People use rdiff-backup in many different environments. The filesystem they want to back up may be on Linux, Windows, or Mac. It may or may not be case sensitive, support characters like ":", have resource forks, extended attributes, or access control lists. Moreover, the file system they are backing up to may or may not support these features.

rdiff-backup tries to handle these situations automatically without the need for switches like --acl --ea --no-ownership, etc. When run it will run tests on both the source and destination filesystems to see what features each supports like case sensitivity, changing uid/gid ownership, resource forks, extended attributes, or access control lists. To see the results of this testing, run rdiff-backup with verbosity 4 or higher, as in -v4.
Mac OS X resource fork support: On Mac OS X systems, rdiff-backup will backup the resource forks which store, for instance, Finder information. Most unix backup programs would only backup the data forks and discard the resource forks.
ACL and EA support: If rdiff-backup can find the pylibacl and pyxattr (mac version) modules, and if the file system supports these features, rdiff-backup will preserve Access Control Lists and user-level Extended Attributes. ACLs are not supported, however, on Mac OS X or Windows as those systems do not use standard POSIX.1e access controls.

Keeps statistics: After each session rdiff-backup writes summary statistics to a text file. You can inspect these to see how large your repository is, how fast it is growing, and how much space rdiff-backup is saving you, and more. Here is an example session_statistics file:

StartTime 1124018521.00 (Sun Aug 14 06:22:01 2005)
EndTime 1124019454.64 (Sun Aug 14 06:37:34 2005)
ElapsedTime 933.64 (15 minutes 33.64 seconds)
SourceFiles 975715
SourceFileSize 13078345389 (12.2 GB)
MirrorFiles 975604
MirrorFileSize 13076177922 (12.2 GB)
NewFiles 119
NewFileSize 1103075 (1.05 MB)
DeletedFiles 8
DeletedFileSize 190653 (186 KB)
ChangedFiles 2032
ChangedSourceSize 395324417 (377 MB)
ChangedMirrorSize 394069372 (376 MB)
IncrementFiles 2233
IncrementFileSize 6098156 (5.82 MB)
TotalDestinationSizeChange 8265623 (7.88 MB)
Errors 80

rdiff-backup also saves very detailed statistics in a file_statistics. This file is also in (compressed) text form, but is usually too voluminous to read manually.