[CentOS] evaluating backup systems: rsync
Gordon Messmer
yinyang at eburg.com
Sat Jan 19 19:20:04 UTC 2013
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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