thekendog The automatic backups aren't running and I couldn't restore files from older ones.
I think I figured out part of the issue on my own. The used storage on my backup server appears to have doubled after v12 and I think it filled up. There's a backup-subvolume folder within each site backup folder and it's the same size as the site itself. For instance, in one my my site backup folders I have a backup-subvolume folder that is 33GB in size AND I have a snapshot-* folder that is also 33GB.
https://share.cleanshot.com/tm4yfPHC
After running apt update && apt install ecp-core and v12-upgrade fix-backup-permissions /backups on your backup server, run all the useradd and chown commands it shows on the backup server. This can take a while on huge backups or slow disks.
If you've done that already, you might need to re-run v12-upgrade update-all-website-backup-locations on your Control Panel server. I say re-run, because this fixed it on my estate, and I checked my bashrc history to confirm I ran this during the upgrade process already. Not sure why it needed to be re-run. Once this was run, the backups started flowing automatically.
BTRFS subvolumes/snapshots aren't used anymore, and should get deleted as your backups get too old to retain. All that should remain are new 'snapshots', which aren't btrfs-snapshots. The used storage goes up because of a bug mentioned by Adam, something with separate mail and app/db servers. All backups will be full backups rather than differential backups.
Restoring is another bug that is being worked on, reported by several people.
