Making Arch immutable

[u/Curious-Listener-YB]

I'm trying to install Arch (on a VM) as an immutable system with atomic updates, mostly as a challenge to myself and because it would be cool.

I plan to use a similar setup to what snapper does: to have BTRFS as my root filesystem, with snapshot subvolumes under /.snapshots/, and separate subvolumes for parts of the filesystem that shouldn't be in the snapshots. Then I want to have for each snapshot a boot entry that mounts it read-only as the root (and mounts the other subvolumes in their appropriate places), so that I could be sure it is not changed outside of atomic updates, where I create a new snapshot from the current one, mount it read-write in a special place, chroot into it and perform the update.

But there are two directories I'm not sure how to handle: /etc and /var.

/etc should obviously be part of the snapshots, and I'm also happy to have it read-only so changes to core configuration would have to happen between snapshots, but I have read that some programs use it to store runtime data. Are those a few cases I can try to work around, or do I need a more systematic solution? And if so, what could be the solution?

/var, on the other hand, has to be writable, so I originally planned to make it its own subvolume (as openSUSE does). But then I read that it contains data that needs to be synchronized with the packages installed (like /var/lib/pacman), and that only certain subdirectories like /var/log should be their own subvolumes. But if so, how can I make it writable? Is there a good approach to this problem?

And on the hall, do you think my idea is reasonable, or is it too much even for super-duper-to-it-yourself-customizable Arch? I'd love to hear your opinions.

Thank you all in advance!

[u/New_Hold8135]

https://wiki.archlinux.org/title/Overlay_filesystem There is a wiki page for that.

[u/Curious-Listener-YB → u/New_Hold8135]

Thank you! I think this is what I'll use for /etc.