A: Microsoft has a whitepaper at its website that details the performance of hard disks when used with Hyper-V. This paper also outlines the performance improvements made to dynamically expanding virtual hard disks (VHDs) between Windows Server 2008 and Windows Server 2008 R2. Looking at the three main types of storage-- pass-through, fixed-size VHD, and dynamically expanding VHD--the ranking of performance seems to be as follows:

  • Pass-through disk and fixed-size VHD perform the same and give the best performance.
  • Dynamically expanding VHD is only slightly slower than the above.

Although pass-through and fixed-size VHD are the same performance-wise, remember that VHD gives better mobility and the ability to use snapshots, which isn't possible with pass-through storage. Consider the new 64TB VHDX in Windows 2012-- the previous 2TB limit of VHD is no longer a reason to use pass-through storage.