I don't know if defragmenting would actually accomplish anything.

Since iSCSI LUNs are virtual block IO devices, the seek time optimization which a defragmenter provides would be of little value, since the chance that the read/write head on the HDD devices in the SAN being at the correct position for the next IO request would be about nil.

So, the time spent by the defragmenter to group the disk blocks for access performance would simply generate a lot of SAN disk IO would have no real effect.

Further, all defragmenters work on the basis of that there is no Read cache for the virtual disk. Almost all RAID controllers provide read cache, some even provide write cache. Finally, most SAN software provide read cache functionality. So, if you have predictive read cache the benefit of defragmenting is almost nullified.