There are two types of fragmentation: file fragmentation and free space fragmentation. File fragmentation concerns computer disk files that are not whole but rather are broken into scattered parts, while free space fragmentation means that the empty space on a disk is broken into scattered parts rather than being collected all in one big empty space. File fragmentation causes problems with accessing data stored in computer disk files, while free space fragmentation causes problems creating new data files or extending (adding to) old ones.
This means that if you ever delete a file say of 64K and then save a file of 70K, part of this 70K file may get saved in the space freed up by deleting the previous 64K and the rest will be saved in some other space that is free causing fragmentation. This is the way the Operating System works whether the Disk Space is composed by a single disk or multiple disks.
So yes, you need to Defrag in a RAID system.
