If you are using ext3 or reiserfs, you don't need to defrag to begin with. Back ups would be good though, in case your drive were to fail.
If you search around, you will find a lot of gurus saying you do not need to defrag. This is Linux, not windoze. No defraging, no viruses, no rebooting to speed up your rig or any of that crap. Just run it and be amazed.
Later
If you really do a search, but my name in the user name field and search for defrag. There should be a thread with some links in it you can read.