Suddenly cannot change metadata profile any longer

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

Here's a new one (with kernel 3.18.7++):

$btrfs --version
Btrfs v3.18.2 (also tried 3.19-rc2)

$mount /dev/sdf1 /mnt/usb

$btrfs filesystem df /mnt/usb
Data, single: total=8.00MiB, used=256.00KiB
System, single: total=32.00MiB, used=16.00KiB
Metadata, single: total=1.00GiB, used=112.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B

Note: metadata=single - done right after mkfs for the initial balance.

$btrfs balance start -mconvert=dup /mnt/usb    
Done, had to relocate 2 out of 3 chunks

root>btrfs filesystem df /mnt/usb              
Data, single: total=8.00MiB, used=512.00KiB
System, single: total=32.00MiB, used=16.00KiB
Metadata, single: total=1.00GiB, used=192.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B

Wait, what? Still single?

This used to work, but somehow no longer does (verified on another
filesystem). If it's dup, it stays dup; if it's single, it stays single.

What works is the very first balance after the mkfs - it successfully
changes from the initial default dup to single, but subsequent balances
don't work. They also don't really fail.

Any ideas what might have caused this? Must have been a recent patch.

Holger