Btrfs disk upgrade done wrong

I'm in the process to upgrade an existing Btrfs multi-disk filesystem (/var/mnt/datapool) and I made a mistake...

I went with a naive:

btrfs device add /dev/sde /var/mnt/datapool/

followed by:

btrfs device remove /dev/sda /var/mnt/datapool/

which was not totally wrong, and add the new disk and I remove the old one. Btrfs is now rebalancing the blocks from /dev/sda to/dev/sde. But it's also bloody slow, only 400MB have been moved after 6h.

I can track the progress with:

btrfs filesystem usage /var/mnt/datapool

And Btrfs prints some event in the Kernel logs that I can track with:

journalctl -exfk

Note for the Gonéri from the future, use btrfs replace next time! It will do (pretty much) a directly block by block copy of the old disk to the new one and it's much faster.