I recently ran into a Microsoft Article indicating that it IS advisable to defrag a Clustered Shared Volume especially if that disk is housing Virtual Machines with Dynamically Expanding Disks.
Unfortunately the answer is more complex really relates to WHERE that CSV is stored. If it is stored on local disk or using Direct Attached Storage then you might want to defrag BUT if it is stored on a SAN (i.e. you are connecting your servers to disk using iSCSI, Fiber Channel,…) that hardware almost certainly takes care of the disk optimization and you should definitely NOT defragment your disk.
To confirm this I posed the following question to Dell EqualLogic:
I have many VM’s using Dynamic Disk on a Dell EqualLogic P4210 iSCSI connected SAN but I thought that the Eql 4210 did its own defrag’s and Windows tools were not to be used.
You should not defrag anything using EqualLogic as storage. The organization of data on the disks is handled by the array, the OS has no control over it. Best case, nothing happens. Worst case, this will cause a lot of page movement and lead to a performance decrease while the disks reorganize all the data and then re-optimize it.
On a related note, you might be wondering how to run a CHKDSK on SAN and we have the answer HERE.