[arch-general] Btrfs snapshots for upgrade operations

[Yclept Nemo]

> create a new subvolume as a snapshot of /
> mount the new subvolume on /mnt/
> pacman -Syu --root=/mnt
> mark the new subvolume as the default one

> In the initramfs, you need an emergency facility to let the user list
> the available subvols (btrfs su list /) and choose one to mount (which
> would require adding subvol=/foo/bar or subvolid=12345 to the
> rootflags).

> And don't forget about /boot, or any other relevant btrfs mountpoint.

How much space does a subvolume/snapshot require?

Will you snapshot all btrfs volumes, or just system-upgrade related
one. For example, /home and /tmp vs /var and /etc. Also this
necessitates a subvol flag per partition - could be messy. Or is it a
bad idea to allow to mix &match upgrade subvolumes - which would
require keeping all the subvolume IDs synchronized?

It seems logical to swap the order - create a snapshot of the current
system which will then be upgraded - especially for security upgrades
which need to be effective immediately, unless it is possible to
remount a volume without rebooting.

It seems that *pacnew and *pacsave files will no longer be required.

Just some thoughts,
Yclept Nemo