btrfs replace fails with unknown error

[sten0]

Hi,

I'm forwarding Debian Bug #866734 where Russell Coker reports that the following fails

# btrfs replace start -B 3 /dev/sdd2 /
ERROR: ioctl(DEV_REPLACE_START) on '/' returns error: <illegal result value>
# btrfs replace start -B /dev/sdb2 /dev/sdd2 /
ERROR: ioctl(DEV_REPLACE_START) on '/' returns error: <illegal result value>
# btrfs replace start -B /dev/sdc2 /dev/sdd2 /
ERROR: ioctl(DEV_REPLACE_START) on '/' returns error: <illegal result value>
# btrfs replace start -B 1 /dev/sdd2 /
ERROR: ioctl(DEV_REPLACE_START) on '/' returns error: <illegal result value>

In this user's case, ae45af7 doesn't seem to be enough, because it doesn't handle the paused balance case. Coke additionally requests that btrfs replace provides a useful error message whenever any of "add/delete/balance/replace/resize" cause a replace to fail.

Best,
Nicholas