release upgrade, and snapper

[jlittle]

(I use snapper, because I like the automatic snapshots. I've recovered from screw-ups nearly painlessly that manual snapshots would not have anticipated.)

I release-upgraded to Cosmic, by doing the obvious, given the root is mounted at /mnt/top:

sudo btrfs subvolume snapshot /mnt/top/@_bionic /mnt/top/@_cosmic
sudo btrfs subvolume snapshot /mnt/top/@home_bionic /mnt/top/@home_cosmic

(If I'd wanted to be super careful about data consistency, I would have done this not booted into Bionic.)

Then I edited /mnt/top/@_cosmic/etc/fstab to refer to the new subvolumes, and edited my grub.cfg to have an entry that starts the copied install. Reboot into it, and run sudo do-release-upgrade, and I'm running Cosmic, but I can go back to Bionic whenever. As well, do-release-upgrade trampled on my EFI boot order, and I had to put it back with efibootmgr.

But snapper failed to start. I'd missed the .snapshots subvolumes that snapper uses. I needed:

sudo rmdir /.snapshots
sudo btrfs su create /mnt/top/@_cosmic/.snapshots
sudo rmdir /home/.snapshots
sudo btrfs su create /mnt/top/@_cosmic/.snapshots

Now it's sweet. I'll have to clean up the bionic snapper snapshots manually.