Re: evaluating backup systems: rsync

[Gordon Messmer <yinyang@xxxxxxxxx>]

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.
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