Btrfs devel - Jan 2011 - mv across subvolumes temporarily needs a lot of space

I was just moving a 40 GB directory across subvolume boundaries, that is
/mnt/extern/mac to /mnt/extern/backup/homes/mac backup being a
subvolume.
I noticed that df will start to show more used space (starting at 130 GB
going slowly to 165 GB for the whole disk) then shortly after the
operation is done it will show 130 GB again.
So I was wondering will I always need as much as 80% of free space
available to move some big directory or is it just wrong information by
df?
Greetings Niklas