how much space does that take up for you?

At least for btrfs, very little.

It's also almost instantaneous.

You can think about it like this: the snapshot creates a bunch of hard links to the existing files. So every file basically has 2 pointers to it.

The upgrade now updates the files in the non- snapshot paths, creating new files/keeping the snapshots on the original file. (That's afaik why it's called a CoW filesystem - copy on write)

Now, every changed file will be stored "twice", but all unchanged files are still only on the disk once.

As most upgrades still keep the majority of all files unchanged, there is effectively little space used for the snapshot

definitely going to check that out, thanks

OpenSUSE has this sort of thing configured by default using BTRFS and snapper. Automatically creates copy-on-write snapshots before and after every time you use the package manager. I think this should be the default on most end-user oriented distros.

Fwiw there's a really nice write-up on how Steam Deck does A/B updates, using rayc and casync/desync. https://github.com/randombk/steamos-teardown/blob/master/doc...

Also Collabora had a nice update on tuning work they did. https://www.collabora.com/news-and-blog/news-and-events/stea...

I'm a little surprised they use casync and multiple partitions, given that root is on btrfs. As they have it is more reliable if anything does go bad, so makes sense, but: given that steam decks are on a given snapshot of the btrfs system, it seems like they could just send an incremental update to the next release and try to boot that. Oh ah, that breaks down if, like me, you disable the readonly root.