Use rsync's checksums to deduplicate across backups

[Alex Waite]

>
> Check out http://backuppc.sourceforge.net/, it's perl-based backup tool,
> using rsync and doing exactly what you ask for.
>
    I have looked at BackupPC before (and it is a nice piece of
software), and it does hardlink across all backups, but believe it
does its own checksum on top of what rsync already does.  I imagine
this would make performance noticeably worse than what I currently
have, though I could be wrong.

---Alex