By default we all (and should) use virtual hard drive (VHD) files to run virtual machines in a Hyper-V environment. But Hyper-V based virtual machines can also attach itself directly to physical disks (pass-through disks). Following are the factors to be considered before deciding to use pass-through disks.

• Studies have shown that VHD performance can be as high as 97% of native disk performance in terms off IOPS (http://www.qlogic.com/promos/products/hyper-v.aspx ). So the question is do you really need to use pass- through disks to achieve high performance or can you optimize the performance by following the best practices around VHD
• Pass-through disks do not support virtual machine snap shots, dynamically expanding hard disks and differencing hard disks
• A single VHD has a maximum capacity limit of 2TB.  Size of a pass-through disk can be bigger than this