For a practical use-case I’ll try out later: The Steam Deck actually mounts /etc with the A/B installed /etc as a lower layer (read-only) with a writeable upper layer for user configs.

In theory you could probably install persistent packages into /etc/extensions/$pkgname with the caveat of having to manually upgrade each package install by hand. Maybe some weird edge-cases here but sounds pretty neat.

Oh, and in case you wonder, all of this only works with distributions that have completed the /usr/ merge. On legacy distributions that didn’t do that and still place parts of /usr/ all over the hierarchy the above won’t work, since merging /usr/ trees via overlayfs is pretty pointess if the OS is not hermetic in /usr/.

Is this purely choice of the systemd-sysext developers or is there a reason they couldn’t provide the same overlaysfs at the root /?