On 2/23/2011 10:39 AM, John Goerzen wrote:
>
> Well, I don't think that's entirely accurate.  BackupPC is on the order of 10
> times slower than alternatives making full backups after the first.

Full backups are understandably slower because the target side is doing 
a full read of all files, and the server side is uncompressing and 
computing the matching block checksums - unless you have checksum 
caching on and the file has been backed up twice.

> It is on
> the order of 20-50 times slower than alternatives making incrementals.  This
> referring to the same disk setup.

This part doesn't make sense to me. You mentioned that you saw it 
opening existing files with strace.  I don't think that should happen if 
(a) the file exists in the previous full backup and (b) the file's 
timestamp and length have not changed.  Are you sure these two things 
are true for the files in question?  Note that large slowly growing 
files or databases with small changes can be problematic, since the 
rsync mechanism will duplicate the target file with a combination of 
uncompressing the old and merging changed blocks from the new - which is 
often slower than just copying the whole thing over again.

One other thing is that rsync will transfer the entire directory listing 
and hold it in RAM while doing the comparisons.  This might be a problem 
with a very large number of files and a small amount of RAM.


-- 
   Les Mikesell
    lesmikesell AT gmail DOT com



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