How defragmentation works in linux filesystems ?

[Sankar P <sankar.curiosity@gmail.com>]

Hi,

Is there any technical article explaining how defragmentation works in
linux filesystems, say btrfs or ext4 ?

Do they recreate the file data blocks and change the root pointer to
the new extent ? Or do they do some kind of moving blocks around ? Or
is it based on some other strategy ? Partial defragmentation (based on
either byte offset or extents) is also supported by any of the file
systems ?

Is there a standard way to trigger a defragmentation operation (an
ioctl ?) that my filesystem could implement so that any user space
tool that work with other fs will work with mine too ? (Like, how
FIEBMAP ioctl can help in giving the extent information for a file
from the userspace)

I googled a bit to find any articles explaining this. But could not
get anything the design. So links to any documentation, article
explaining the linux filesystems' way of defragmenting are welcome.

Thanks.

--
Sankar P
http://psankar.blogspot.com