Why does btrfs defrag work worse than making a copy of a file?

[Sebastian Ochmann <ochmann@informatik.uni-bonn.de>]

Hello,

I have a VirtualBox hard drive image which is quite fragmented even 
after very light use; it is 1.6 GB in size and has around 5000 fragments 
(I'm using "filefrag" to determine the number of fragments). Doing a 
"btrfs fi defrag -f image.vdi" reduced the number of fragments to 3749. 
Even doing a "btrfs fi defrag -f -t 1 image.vdi" which should make sure 
every extent is rewritten (according to the btrfs-progs 3.14.2 manpage) 
does not yield any better result and seems to return immediately. 
Copying the file, however, yields a copy which has only 5 fragments 
(simply doing a cp image.vdi image2.vdi; sync; filefrag image2.vdi).

What do I have to do to defrag the file to the minimal number of 
fragments possible? Am I missing something?

Kernel version 3.15.5, btrfs progs 3.14.2, Arch Linux.

Best regards,
Sebastian