I was using Allway Sync for backing up my music library. I needed versioining because earlier I had problems with Winamp when I was updating metadata of the files and sometimes I think it corrupted the whole files.
But Allway Sync versioning is too simple, it only takes a copy of the whole file so even when I was changing one character of metadata the whole file was copied to version history.
I realized I need "reverse delta" - capable backups.
At first I was thinking about setting up Subversion, importing music library there and then mounting that library as WebDav mount. But that felt as too heavy solution. I still needed to have the music library in at least two different drives.
I tested how much the files change when you change only the metadata. I changed two metadata fields of mp3 and flac files. In mp3 file about 65K bytes where changed because of this but in flac file only about 200 bytes were changed. In ogg files 99% of bytes were changed so it's not a good format for reverse delta. I guess ogg's metadata is compressed along with music data.
I decided to go with KISS principle and installed rdiff-backup. It's command line is simple enough: rdiff-backup source_dir dest_dir
It does reverse delta and you can restore the backup to certain time. Now it's scheduled to run every night.