I'm playing a little bit with LVM. I've noticed that I can add a new HDD, include it in the volume group, and add some space to all of the existing volumes without unmounting theirs filesystem or rebooting the whole machine. But when I tried to do the opposite thing, i.e. to reduce the volumes' space and remove the drive from the system, I was unable to do it online. This can be achieved when the filesystems are unmounted, or from live cd/dvd/pendrive (root filesystem).
So the question is, why the filesystem can't be shrunk online?