I'm working with remote backups of large files, and found a solution that
can be used when the backup file is quite similar to the previous one.

I think that it could be used to send by rsync large backups of virtual disks.

Basically, you have to:

* gzip --rsyncable today_backup_file (very important to use --rsyncable)
* rollcut -d 65536 today_backup_file.gzip
* dupemerge backup_folder (deduplication using hardlinks)
* use backuppc and rsync to remote backup your backup_folder

Rollcut is a tool that I've done to cut similar large files, into several small
chunks with a hig probability of some being equal.

After using rollcut, you deduplicate all the small chunks, using hardlinks.
So, every chunk that is the same of one of the previous ones, will be
hardlinked locally.

As rsync knows about hardlinks, after the first backup all the hardlinked
chunks will not have to be send over the network.

You can find more information, and download rollcut in 
https://github.com/ferfebles/rollcut


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