Les Mikesell <lesmikesell <at> gmail.com> writes:

> 
> Shawn Perry wrote:
> > Take a look at how Unison does it's compares.
> > 
> 
> It's not impossible - it just can't be done with existing tools 
> and storage scheme.
> 

This look indeed like a complicated problem. 

There is an rsync patch which add an option --link-by-hash=DIR pretty similar
to the pool of backuppc. It create hard links of similar files ordered by 
 md4 sum on the fly during the sync process. I tested it and I'm looking
at the C code right now. 


I'm sure there is less than 5 lines to make it skip a remote file whom the hash
match a file in the pool. It imply a patched rsync. That may be acceptable for
precise cases like mine where bandwidth is critical but only on the backuppc
side of rsync (the client). This mean the rsyncd part will stay regular rsync
and send sums of the whole files not following BackupPC_Link logic. So It would
imply in turn to change the way BackupPC_Link manage the pool, etc... And there
is also the duplicate hash problem.

My better idea so far would be to use a patched rsync and a separate pool
directory for rsyncd method only. Matching against this pool would be done only
if the classic matching by path failed. The matching file would also not be
just hard-linked but rather serve as base file for rsync update, so in case we
have several candidate one can safely use the first coming file. This way it can
be integrated in BackupPC only with some host configuration.

Malik.


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