Time Machine automatically nuking snapshots

[vimalloc]

It looks like time machine will occasionally nuke snapshots itself. If I had to guess, it is probably doing something along the lines of tmutil thinlocalsnapshots /.

As it stands, the only way I found to create APFS snapshots is via tmutil, so unless whatever is deleting the snapshots we created is run on a cron or something that we can disable, there isn't much to be done about this. Ideally we would have a C API or a different set of tools outside of time machine that let us create APFS snapshots, but until that is a thing we will take what we can get.

This won't cause the program to crash, if it finds that a snapshot was removed out from under it, it will just remove that snapshots metadata from the database and keep happily running. But it does mean that I cannot guarantee the amount of snapshots to keep limits found in the config file will be honored.

[vimalloc]

Doing testing with an actual snapshot schedule, it looks like it doesn't let me keep more then 4 or 5 snapshots at a time, which equates to never having hourly snapshots kept. This is super unfortunate, but unless apply provides a way around this i don't see what could be done.

[vimalloc]

It looks like there is a way to create snapshots via a C syscall:

#include <fcntl.h>
#include <stdio.h>
#include <sys/attr.h>
#include <sys/snapshot.h>

int main() {
	int dirfd = open("/", O_RDONLY, 0);
	if (dirfd < 0) {
		perror("open");
		return(1);
	}

	int ret = fs_snapshot_create(dirfd, "snapshot_test", 0);
	if (ret != 0)
		perror("fs_snapshot_create");
	return (ret);
}

However, running this, even as root, returns the error fs_snapshot_create: Operation not permitted. Looking at this thread (https://forums.developer.apple.com/thread/89635), it looks like we would need a special entitlement to get that to work, and based on what that sounds like I highly doubt apple would give me one (stupid apple). But, if I have some more time, I will see if it is possible.

[leftytennis]

Did you find any more info regarding the entitlement required and how to request this?

[Jip-Hop]

I havent tried this myself, but when reading the snapshot documentation of CCC, it seems they have found a way around this. The interface distinguishes between snapshots made by CCC and snapshots made by Time Machine. Also it guarantees the following:

† CCC applies a more conservative retention policy for the startup disk — weekly snapshots are not retained, and daily snapshots are only retained for 7 days. You can customize those settings if you want a longer retention for snapshots on the startup disk, but be sure to consider the implications this will have on disk usage on your startup disk.

[Jip-Hop]

I think I found the answer here:

Time Machine should only monkey with its own snapshots. If you create a snapshot via other means, Time Machine should leave it alone. As far as creating such a snapshot, that requires a special entitlement as per this thread. Share and Enjoy

Seem like the key is to use man fs_snapshot_create instead of tmutil.

[Jip-Hop]

Whoops. I now see you already posted that article on 11 Nov 2017. I guess CCC was able to get this special entitlement because they're a backup application developer.

[leftytennis]

Seems really odd to me that this special entitlement is required at all and not just require elevated privileges for the effective uid...

[vimalloc]

Seriously, it's super dumb 👎

[sclsj]

You can mount the snapshot somewhere, and Time Machine won't be able to delete it. I've tested mounting multiple snapshots and it did not create any problems.

[sclsj]

Another option: use snaputil. See here: https://github.com/ahl/apfs/issues/2.