fails to convert meta/system directly from RAID1 to DUP

btrfs balance fails when trying to convert metadata or system chunks to dup profile on filesystems with multiple devices. This requires that a conversion from a multi-device filesystem to a single-device one go through a state of no redundancy for a brief period.

This was discussed two years ago in http://linux-btrfs.vger.kernel.narkive.com/KpQhMabL/patch-btrfs-allow-forced-conversion-of-metadata-to-dup-profile-on-multiple-devices and apparently only recently fixed in upstream 4.6 according to #btrfs. Yakkety is still on 4.4 and we should look into backporting the patch for xenial.