Why do I have to remount to receive?

Version information:

  # btrfs --version
  Btrfs v3.17
  # uname -a
  Linux maru2 3.16.0-4-amd64 #1 SMP Debian 3.16.7-2 (2014-11-06) x86_64 GNU/Linux

In the following system, /backup is a mounted subvolume. The initial send/receive worked, but when trying to send an incremental I get the following warning:

  # btrfs send -p /snapshots/2014-12-09 /snapshots/test | btrfs receive -vvv /backup/snapshots/root/
  At subvol /snapshots/test
  At snapshot test
  receiving snapshot test uuid=9c72c30a-04c0-6442-9a9a-7ee9c56014e2, ctransid=1030360 parent_uuid=4eb30116-2c1a-f94d-b964-98cd41262dfb, parent_ctransid=1030202
  It seems that you have changed your default subvolume or you specify other subvolume to
  mount btrfs, try to remount this btrfs filesystem with fs tree, and run btrfs receive again!

I understand what it is saying, but I have to ask, why? Why does it or should it need me to remount the root fs to do an incremental receive?

Michael Welsh Duggan
([email protected])