All of these have status R and D for their duration, and while all get
a SIGKILL from systemd on logout, none of the processes change status
or die until their kernel task is done. And each of these operations
complete successfully with no worse for the wear.

btrfs balance &
btrfs dev rem &
btrfs replace start

Only 'btrfs scrub' has status S, and once it gets SIGKILL, it goes Z
and all of its accounting is wrong. But the kernel tasks continue and
appear to complete.

I did all of this with a btrfs raid5, 3 and 4 disks, in a libvirt VM.

---
Chris Murphy