What is the current status of defragmentation?

[Adam Ryczkowski <adam.ryczkowski@statystyka.net>]

Today I booted my linux-mint 14 into the latest 3.10.1 kernel to
defragment the root btrfs filesystem on my ASUS N56VZ laptop with hybrid
Seagate Momentus XT disk. I did something like

   find / -exec btrfs filesystem defrag {}

To my amazement it didn't really made a lot of I/O (the HDD LED wasn't
blinking). Confident that at the beginning the defragmentation is
CPU-bound (like it sometimes is for instance in case of O&O Defrag on
Windows world), I let it go on, and I switched myself to my other
chores. After a few minutes my system froze. I was barely able to switch
to text terminal and login, but I couldn't reach bash prompt; it looked
like the OS couln't perform any I/O on the disk. At the end, the system
responded to SysRq REISUB combination though; Unfortunately I can't
reproduce exactly what I typed in, because the command was not logged to
the bash history...).

It clearly showed me, that defragmenting the filesystem is not that
trivial, as it is for ext4. So I have quesions:

* Is the defragmentation of the whole filesystem supported at all? I
can't find a single reference that it is, and a syntax of btrfs-progs
suggest that it isn't. If supported, under what conditions? Like what %
of free space should be available?

* How to check the level of defragmentation, and what are the reasonable
threshold values, that should indicate the desktop filesystem needs
defragmenting? I know, that everyone's millage my vary; I just want to
know some values as a point-of-reference.

* What is the recommended command, that would efficiently defragment the
whole file system, preferably with some sort of progress indication?
Does this command

find / -type f -o -type d -print0 | xargs --null --no-run-if-empty btrfs
filesystem defragment -cv

look like a reasonable idiom for defragmenting the whole filesystem?

Thank you for your help,

Adam Ryczkowski