Why so much "btrfs send" data for "cp -a --reflink"?

[Torsten Bronger <bronger@physik.rwth-aachen.de>]

Hallöchen!

I have two subvolumes A and B.  „A“ contains 50GB data, B is empty.
None is a snapshot of the other.  Now, I copy all data from A to B
with "cp -a --reflink A/* B".  This copying takes less than a
second.  So apparently, no bulk data was duplicated.  "diff -rq A B"
is empty.  So far, so good.

However, it surprises me that

    btrfs send -p A B | wc -c

reports 12GB.  I would have hoped for very few data (say, a couple
of MBs).  Or, the whole 50GB (because A is not a real parent of B,
and never has been).  An additional "-c A" does not change anything.

Why is this?  In other words, what comprises those 12GB?

It may be insignificant, but the 50GB are almost fully a single
VirtualBox .vdi file, somewhat fragmented (filefrag says 16000).

Regards,
Torsten.

-- 
Torsten Bronger