[systemd-devel] Systemd loads units before btrfs subvolumes are mounted

[Andrei Borzenkov]

25.05.2016 18:21, Lennart Poettering пишет:
> 
> What I don't understand in your case though is why you use an explicit
> mount for /opt anyway... The nice thing about btrfs is that subvolumes
> behave pretty much like normal directories that just have a bit more
> features. And because of that there's no need to mount them
> explicitly. Just create a subvolume normally and everything should be
> good, it will then be available at the right place the instant the
> parent subvolume is mounted too.
> 


As soon as you revert to different snapshot (by pointing root to it) you
lose all subvolumes that are referenced from your initial snapshot. And
ability to quickly revert to recover from serious failure is one of
btrfs selling points. Or in other direction - you clone your root, apply
updates to it and reboot into newly created clone.

So in practice your suggestion does not work (except in very trivial
configuration) and every volume must be separate mount point to make
sure it is universally available.