Ext3 and RAID: silent data killers?

Since I've researched this a lot recently, here are some rsync/librsync based tools that work somewhat like version control systems but are intended for system backups. They qualify as 'near-CDP' since rsync is efficient at scanning for changes.

rsnapshot is pretty good as a 'sort of' version control system for any type of file including binaries. It doesn't do any compression, just rsync plus hard links, but works very well within its design limits. It can backup filesystems including the hard links (use rsync -avH in the config file), and is focused on 'pull' backups i.e. backup server ssh's into the server to be backed up. It's used by some web hosting providers who back up tens of millions of files every few hours, with scans taking a surprisingly short time due to the efficiency of rsync. Generally rsnapshot is best if you have a lot of disk space available, and not much time to run the backups in.

rdiff-backup may be closer to what you are thinking of - unlike rsnapshot it only stores the deltas between versions of a file, and stores permissions etc as metadata (so you don't have to have root on the box being backed up to rsync arbitrary files). It's a bit slower than rsnapshot but a lot of people like it. It does include checksums which is a very attractive feature.

duplicity is somewhat like rsnapshot, but can also do encryption, so it's more suitable for backup to a system you don't control.

There are a lot of these tools around, based on Mike Rubel's original ideas, but these ones seem the most actively discussed.

For a non-rsync backup, dar is excellent but not widely mentioned - it includes per-block encryption and compression, and per-file checksums, and is generally much faster for recovery than tar, where you must read through the whole archive to recover.

rdiff-backup, like VCS tools, will have difficulty with files of 500 MB or more - it's been reported that such files don't get backed up, or are not delta'ed. Very large files that change frequently (databases, VM images, etc) are a problem for all these tools.