So bear with me here, I have an idea.
I have mounted /home in fstab and a mysterious subvolume to /home/username and then my archived subvolume. So far so good.
From a few experiments, I would very much have the archive subvolume at zstd:12. I almost never write to it (once every few months, I can wait for the copy, no problem) and reading doesn’t need to be realtime.
As the root volume ( here /home) is mounted with zstd:3, couldn’t I “just” for copying files to the archive remount the full volume? In TTY, this should be relatively easy, right?
# On TTY1
sudo umount /home
sudo mount -o defaults,noatime,compress=zstd:12 /dev/mapper/crypthome /home
# On TTY2
rsync -av --info=progress2 /home/username/project/a /home/username/archive/
logout
# On TTY1
sudo umount /home
sudo mount -a
Or am I totall out of place?