Hyper-V Pass-through Disks: Advantages, Disadvantages, and Limitations

Hyper-V Pass-through Disks may be the key for better performance; however, they come with their own disadvantages and limitations, too.

First of all the speed gain of using Hyper-V pass-through disks is probably overrated when compared to a fixed-sized VHDX. Estimates are that a pass-through disk may be around 15% faster due to a reduction of overhead.

Another great thing besides speed is you can run a VM using a physical partition with an OS already on it for a test.

The issue with pass-through disks in Hyper-V is that you can't easily move them around. You can't just copy them to another folder or host using a plain file copy, as you could with VHDs.

The second major issue has to do with backups. Backups can't run on the host when passthrough disks are involved. The VM has to run the backup instead because only it has exclusive access to its own file system.

When backing up from the host, VSS needs to have control over the file system; however, this is not the case when partitions are handed over directly to the Hyper-V VM.

If you had a couple of VMs running on pass-through disks, you'll need backup apps or backup agents inside each of them. In addition, this would be quite inefficient because you would be using virtualized CPUs to compress the backups. It's, hence, much more efficient to run backups from the host. Either way, we recommend using BackupChain Hyper-V Backup, whether on the host or inside a VM.

How to Convert a Pass-through Disk to a VHD / VHDX

run the following command from the PowerShell box:

	New-VHD -Path "C:\MyVHDX_folder\Converted.VHDX" -Dynamic –SourceDisk 2

The above command takes drive #2 and converts it into a VHDX.

Alternatively you could use the New Virtual Disk Wizard in the Hyper-V Manager to create a new disk and then choose the option 'copy the contents of the specified disk' to select the source disk.