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).