Matt,

[If you sent two mails because your first didn't immediately 
arrive: it was probably held for moderation since you appear to be 
new here.  Welcome!]

Matt Huszagh wrote:
> I'm encountering the error "rmdir: Device or resource busy" when 
> I
> attempt to perform guix system init on a mount point that has 
> btrfs
> subvolumes.

So something's (preventatively?) deleting a directory that has 
something mounted over or in it, or is a subdirectory itself 
(although that seems unlikely in this layout).

I had a dig around, didn't find any likely culprits, but this 
could be happening anywhere.  Does ‘guix system init’ print 
anything else?

Don't waste *too* much time debugging your error message, though. 
Guix System doesn't have any code to handle fancy subvolume 
swappery like this.  I'm afraid you'll have to drop the @-scheme 
entirely.

> mount -o subvol=@guixsd,$o_btrfs LABEL=btrfs /mnt
> mount -o subvol=@gnu,$o_btrfs LABEL=btrfs /mnt/gnu

The two above are especially problematic: Guix's GRUB looks only 
for /gnu/store/…-linux/, which is now two indirections away.

Guix System *probably* supports⁰ ‘in-place’ subvolumes like

  btrfs subvol create /gnu/store

only because it doesn't need to support them: they're just 
directories.

> mount -o subvol=@grub,$o_btrfs LABEL=btrfs /mnt/boot/grub

Does GRUB support magically finding itself elsewhere like this? 
Does this work on other distributions?  If so, do you know how?

…

You're damn right that this needs to be improved.  Hands welcome 
:-)

Kind regards,

T G-R

[0]: At least I think I did so once but never made use of its 
subvolliness, so I stopped doing so.