Team Fusion wrote a great post on “defragging” vm’s and why you shouldn’t do it. Or at least be very careful about it. Make sure to read it!
Before we begin, it’s important to note that defragmentation isn’t a necessary task - your virtual machine will still work just fine even if you never defrag, and the effects of fragmentation are usually not noticeable. Personally, I’ve never feel the need to defrag. However, if for some reason you do feel the need to defrag, here’s how to do it. Note that snapshots get in the way of proper defragmenting.
This article relates to sparse disks in Fusion. The main reason they argue not to defrag them, is that it would grow the disk. Obviously this doesn’t apply to most ESX cases. I wonder if the same advice would apply to monolithic disks as well? Anyone know?