On Thursday 29 April 2010 13:44:52 Tyler J. Wagner wrote:
> I use local rsync for backing up the server itself (indeed, for about 70
> servers), and it has always been fine for me. I don't think the
> checksumming really makes much difference, as once it is copied (to
> itself) it will still have to be checksummed to go into the pool.
Yeah, but the rsync protocol's chunk checksumming is designed to reduce
network traffic, and since in the case of a localhost backup the bandwidth is
however many orders of magnitude higher than over a network, it's just going
to waste CPU cycles, isn't it?
I know that when used to sync two local directories, rsync proper will enable
--whole-file by default for this reason. However, because RsyncP is involved,
I doubt rsync will be aware that it's a local transfer, so the option would
have to be specified explicitly.
> Is the checksum caching rsync does the same as the checksum that is stored
> in the pool?
No, it's a different hash. My understanding is that under defaut RsyncP
conditions, BackupPC will do two pooled file decompressions, plus a checksum
for each chunk of the file, plus a whole-file checksum, for each file.
Checksum
caching prevents all of this CPU overhead, but does not affect BackupPC's pool-
related checksumming. BackupPC will still perform file content hashing and
filename mangling regardless.
Here are the docs on checksum caching for reference:
http://backuppc.sourceforge.net/faq/BackupPC.html#rsync_checksum_caching
Paul
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