Unwanted full rescan&resync @ SD card @ Syncthing Android after timezone/daylight saving change

JardaG · December 9, 2023, 7:44pm

Hi, first of all, I want to thank you for taking the time to address this issue. =) As for testing the fix, I have read pull request #9231 and I am very happy to participate in testing, I just don’t have much idea how to do it, especially where to get the right build.

I’d also be interested in the exact implementation of the fix, because when I was going through the changes in the files (I’m not a programmer), it didn’t seem to me, according to the comments, that it would fix traveling between time zones as well, especially those that aren’t offset by exactly whole hours, as I described here.

github.com/syncthing/syncthing

Handle daylight savings time changes with FAT filesystems

opened 05:29PM - 15 Nov 23 UTC

calmh

enhancement

FAT filesystems store timestamps in reference to whatever the computer considers… local time. This means that when time changes due to daylight savings the timestamps all shift by one hour in some direction, causing files to be rehashed and resynced (at least metadata wise). Discussion: https://forum.syncthing.net/t/unwanted-full-rescan-resync-sd-card-syncthing-android-after-timezone-daylight-saving-change/21064 It's of course also possible for "local time" to be UTC or another zone that doesn't shift, or the filesystem can be mounted with `tz=UTC` to store timestamps in UTC. But this is not the most common setup on Windows or Android where FAT is more common. One solution to all this, implemented by OwnCloud and possibly other sync programs, is to ignore timestamp changes of precisely one hour. We could do the same. We even have a good place to do it -- the mtimefs. We already virtualize the timestamp on top of what's actually on the filesystem, we would more or less just need to accept differences of precisely one hour where we check for equality. In theory this means some changes might not get picked up, but it will be rare in practice. On filesystems with better than second timestamp precision it would be essentially impossible to trigger by mistake.

So maybe it’s not a perfect solution, but I’m still very happy that it might help. =)

system · January 8, 2024, 7:44pm

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.