Re: [BackupPC-users] RsyncP and --whole-file

[Paul G. D. <pdg...@go...>]

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