Comment 38 for bug 321528

I found another tool for online defragmentation - shake-fs, see: http://vleu.net/shake/
There are packages for Ubuntu and Debian at http://www.vleu.net/apt/ and https://launchpad.net/~un-brice/+archive/ppa

Shake is a defragmenter that runs in userspace, without the need of patching the kernel and while the system is used (for now, on GNU/Linux only).
There is nothing magic in that : it just works by rewriting fragmented files. But it has some heuristics that could make it more efficient than other tools, including defrag (see ck.kolivas.org/apps/defrag/) and, maybe, xfs_fsr.