WS2012 Hyper-V – Virtual Hard Disk (VHD) Block Fragmentation

The Performance Tuning Guidelines for Windows Server 2012 document is available and I’m reviewing and commenting on notable text in it.

This is a small but important note in the document:

Just as the allocations on a physical disk can be fragmented, the allocation of the blocks on a virtual disk can be fragmented when two virtually adjacent blocks are not allocated together on a virtual disk file.

The fragmentation percentage is reported for disks. If a performance issue noticed on a virtual disk, you should check the fragmentation percentage. When applicable, defragment the virtual disk by creating a new virtual disk with the data from the fragmented disk by using the Create from Source option.

Hmm, I’ve MSFT for more information on this one; I would have thought that we could defrag the LUN that the fragmented VHD was on, rather than create a whole new VHD.  I’ll update this if I get an answer.