support for btrfs multiple devices?

[Matt Huszagh]

I'm unable to get even a basic guix system init working with btrfs on 2
partitions. I setup the btrfs filesystem with:

mkfs.btrfs -L btrfs -m raid1 -d raid0 /dev/nvme0n1p2 /dev/sda1

and in my config:

(define fs-root
  (file-system
   (mount-point "/")
   (type "btrfs")
   (device (file-system-label "btrfs"))
   (options "defaults,compress=lzo,ssd,noatime")
   (needed-for-boot? #t)))

(define fs-boot
  (file-system
   (mount-point "/boot/efi")
   (type "vfat")
   (device (uuid "68C5-A2D1" 'fat))))

 (file-systems (append
                (list
                 fs-root
                 fs-boot)
                %base-file-systems))

When I try to boot into my system I get the error "error: couldn't find a
necessary member device of multi-device filesystem" and then I'm sent to
the grub rescue prompt. It seems like maybe "btrfs device scan" isn't run.
However, looking in the guix source code it seems this is part of the
filesystem check which is enabled by default. Mounting with the label works
fine from the flash drive so I don't think the fs-root definition is wrong.

I've already given up luks encryption and my subvolume layout, hoping that
I could at least get a barebones btrfs setup to work. I would really prefer
not to switch to ext4. Has anyone gotten btrfs working with any sort of
multi-device raid capability? Any pointers would be greatly appreciated!
I've now devoted 4 full days and still can't get past the guix system init
phase - I'm pretty close to giving up...