Take a look at how Unison does it's compares.

On Fri, Dec 18, 2009 at 9:12 AM, Les Mikesell <lesmikesell AT gmail DOT com> 
wrote:
> Malik Recoing. wrote:
>>
>>>> I know a file will be skiped if it is present in the previous backup, but
>>>> what
>>>> appens if the file have been backed up for another host ?
>>> It is required to be uploaded first as otherwise there's nothing to
>>> compare it to (yeah, I know, that's a pain[1]).
>>>
>>> It might theoretically be sufficient to let the remote side calculate a
>>> hash and compare it against the files in the pool with matching hashes,
>>> and then let rsync do full compares against all the matching hashes in the
>>> pool (since hash collisions happen), but I don't believe anyone has tried
>>> to code this up yet, and it would only be of limited uses in systems that
>>> were network bandwidth constrained rather than disk bandwidth constrained.
>>
>> I'm quite sure it will be an improvement for both. Globaly there will be no
>> overhead. More : the hash calculation will be kind of "clustered" delegating 
>> it
>> to the client. The matching of identical hash is anyway done by 
>> BackupPC_Link.
>> Thus BackupPC_Link will became pointless in a "rsync-only" configuration. The
>> disk and the network trafic will be reduced as many files won't be 
>> transfered at
>> all.
>
> There are two problems: one is that the remote agent is a standard rsync
> binary that knows nothing about backuppc's hashes; the other is that
> hash collisions are normal and expected - and disambiguated by a full
> data comparison.
>
>> I tougth of a similar solution. When your client are mostly "full system 
>> tree"
>> backups, you may have ready-to-copy backups of the differents OS tree. When a
>> new client is added, you copy the corresponding OS directory as it was the 
>> first
>> full backup.
>
> Yes, if your remote machines are essentially clones of each other, you
> could create their pc directories as clones with a tool that knows how
> to make a tree of hardlinks.
>
> A better solution might be to have a local machine at the site running
> backuppc and work out some way to get an offsite copy.  If bandwidth is
> such an issue, you are also going to have trouble doing a restore.  But,
> if you've followed this mail list very long you'd know that the 'offsite
> copy' problem doesn't have a good solution yet either.
>
> --
>   Les Mikesell
>    lesmikesell AT gmail DOT com
>
>
>
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