RSync like diff for incremental stuff

Hi,

  I'm wondering if DAR could use rsync like system for incremental backup.
Currently, if I want to backup my mail file (3GB) that changes hourly, 
it back it up everytime it's run (and the whole 3GB).

With a rsync like algorithm, it would have saved only the difference 
(which is not that much, something like 10MB).
Usually rsync requires 2 copies of the file (the "previous file" and the 
"new file"), in order to find out the difference.
This is an annoying requirement for an archive format, as the "previous 
file" is usually compressed / encrypted, and as such, would require too 
much CPU to uncompress / decrypt for comparing.

However, librsync has a mechanism to compute a "signature" of a file, 
and this signature is used in place of the "previous file". The 
signature itself is very small (compared to the file), so it might be 
possible to store such signature in the catalog.
The signature is compared to the "new file", and the diff can be made 
from this (so in my previous example, only 10MB of data would be stored 
in the backup).

Upon restoring however, the whole chain of backup must be read (as the 
final file is made of original_file + diff(s)).
As restoring happen very rarely, I don't think it's a problem, as the 
space gained by using diff worth the extra CPU time.

That way, the DAR format would really, really fit all the possible 
requirement for a backup tool, as it would be optimal in size (rsync 
like algorithm), optimal in speed (binary code), optimal in security 
(encryption).

What do you think about this ?
Cyril


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