On Wed, 2011-01-19 at 17:26 +0000, Tyler J. Wagner wrote:
> On Wed, 2011-01-19 at 16:34 +0000, Ed McDonagh wrote:
> > Isn't someone going to mention checksum caching to save processing on
> > the backup server side?
> 
> Thanks for doing so. Perhaps you could explain?
> 
> Regards,
> Tyler
> 

Not without looking it up, when there are far more authoritative people
on this list who can trot it off without thinking!

However:
http://backuppc.sourceforge.net/faq/BackupPC.html#rsync_checksum_caching

Basically add --checksum-seed=32761 to both the RsyncArgs and the
RsyncRestoreArgs.

Without this option, each file on the backuppc side needs to be
uncompressed and the checksums are calculated to send to the client for
comparison. Therefore there is a lot of disk i/o and processor use
without actually transferring any file data because nothing has changed.

With this option, the checksums are cached on the server side, thus
removing the disk i/o and processing. This still needs to take place on
the client side, but without the decompression step.

There is also a $Conf{RsyncCsumCacheVerifyProb} setting with a default
value of 0.01, which means that 1% of the time the cache checksums are
verified.

Hope that helps.
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