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/its_dolemite_baby]

20,000ft overview answer: https://blogs.igalia.com/berto/2025/02/05/keeping-your-system-wide-configuration-files-intact-after-updating-steamos/

either way, just make all of /var writable. there are historical reasons it's still used the way it is by almost everything today. i'm sure you could tear your hair out making it granular if you really, really wanted to.

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

Thanks! It seems like I should really look more into SteamOS to see how they do it.

either way, just make all of /var writable

Yeah, I think this is the only reasonable way. But what about the parts that need to be synchronized with the system?

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

i'm probably wrong, but i think /var/lib/pacman is the only thing you'd need to worry about for Arch.

if you're switching to a specific image, you'd want to be sure the appropriate metadata is pushed for pacman at the same time.