It is true that UNIX uses a better disk write algorithm and so will fragment more slowly than a Windows machine however fragmentation will eventually affect a UNIX machine especially if there is limited disk space. With UNIX the most effective way to defragment a disk is to back everything up to tape, reinitialize the filesystem with MKFS and then restore the file system from the tape. You need to ensure you back up the files and not a disk image otherwise you will just
restore the fragmentation.