[CentOS] evaluating backup systems: rsync

Nicolas Thierry-Mieg Nicolas.Thierry-Mieg at imag.fr
Sat Jan 19 19:31:31 UTC 2013

Gordon Messmer wrote:
> On 01/19/2013 10:28 AM, Nicolas Thierry-Mieg wrote:
>> Not true: if you change the modification time on a file, by default
>> rsync will copy the whole file again
> rsync uses an efficient algorithm to compare file contents and transfer
> only the differences.  Reindl was correct.  rsync will use very little
> bandwidth in this case.  You can test this by rsyncing a large file from
> one system to another, "touch"ing the file, and then rsync again.  rsync
> will take a little while to generate checksums of the data to determine
> what needs to be copied, but will not transfer the entire contents of
> the file.
> If you run rsync with the -v flag, it will report the saved bandwidth as
> its "speedup".  IIRC, this is expressed as the ratio of the size of
> files which were detected as not matching based on the given criteria
> (mtime and size by default, but possibly by checksum if given -c) to the
> size of data that was actually transmitted.

agreed, except if both source and dest are local, eg back up to a USB 
HD. If you test that you'll see the speedup is 1 (ie no speedup).

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