While a solution using rsync would be ideal, if you don't have a huge
number of files  or lots of huge files which meet this special case, it
probably wouldn't be prohibitively difficult to write your own special
case script.

It could be set to ignore all files that rsync will handle normally (but
it would still have to look at both sides to determine that.)

On 1/22/19 5:57 AM, just subscribed for rsync-qa from bugzilla via rsync
> https://bugzilla.samba.org/show_bug.cgi?id=13735
> --- Comment #3 from Sébastien Béhuret <sbehu...@gmail.com> ---
> Thank you for suggesting the patches repo. An improved checksum/maybe-checksum
> algorithm would be great but there appears to be a lot of work to achieve 
> this.
> Checksums are very handy for special cases (e.g. to detect and fix data
> corruption) but are still relatively slow and prone to collisions or require
> specific patches as you suggested. We ideally want the possibility to enforce
> the synchronization of files that are more recent on the sending side when
> mtime and size are identical on both sides. This would improve the reliability
> of system backup software that are based on rsync, and could be implemented as
> a new option to alter the behavior of the quick-check algorithm.
> Overall, rsync lacks a solid way to detect and transfer back-dated files. I
> feel like the importance of dealing with back-dated files is underestimated:
> In a file system, file back-dating may occur during software updates without
> malicious intent and users being aware of it. An example of file back-dating 
> is
> found in Firefox package in Debian-based distributions. Some JS files in
> /usr/share/firefox/browser/defaults/preferences/ directory are always dated
> 2010-01-01 00:00:00. When changes in these files are small (e.g. a version
> string, a fixed-size series of characters such as a timestamp, hash or key),
> the files end up with the same size and mtime and the changes won’t be 
> detected
> by rsync quick-check algorithm. Backup software relying on rsync for
> incremental updates will eventually get wrong unless they use the --checksum
> option, but this is sub-optimal (and sometimes buggy) and most backup systems
> don’t even allow the user to add this option.
> Quick fix suggestion:
> This may be a bit of an oversimplification, but assuming that the current 
> rsync
> quick-check algorithm looks like this:
> synchronize(source, dest) IF [ mtime(source) != mtime(dest) AND size(source) 
> !=
> size(dest) ]
> Then a new option (e.g. --use-ctime or --ignore-times-if-newer) could alter it
> in the following way:
> synchronize(source, dest) IF [[ ctime(source) > ctime(dest) ] OR [
> mtime(source) != mtime(dest) AND size(source) != size(dest) ]]
> (Notice the use of ‘greater than’ rather than ‘not equal’ to compare ctimes.)
> This would do the trick and ensure that files that were back-dated are 
> properly
> detected and synchronized during incremental updates. I think that such an
> option is a must-have for reliable backup software, and could even be enabled
> by default since atime updates do not alter ctime.

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