Q: Is it true that converting VMDKs to virtual hard disk (VHDX) files is quicker using a NetApp SAN? Why?

A: NetApp has its own complete set of Windows PowerShell cmdlets, and one of these performs the conversion of VMDK to VHD; however, it does it in a different way from most conversion tools.

Typically a conversion tool creates a target VHDX, then copies the data blocks from the VMDK to the VHD file. The NetApp tool works differently--it actually converts the VMDK to a VHDX in-place by essentially just rewriting the metadata and header of the VMDK to make it a VHD file.

The solution still creates a separate VHDX file as output, but behind the scenes it's using its FlexClone technology to clone the data blobs of the VMDK to the new VHD, which means the actual data is not actually copied on the SAN. This results in conversions taking seconds instead of minutes.

You'll find a nice overview of this technology at the NetApp website.

Microsoft actually has a solution with NetApp for VMware migrations that leverages a temporary NetApp storage array at the customer location so the optimized VMDK to VHDX conversion can be used. The solution works as follows:

  1. The VMware virtual machine (VM) is storage-vMotioned to the NetApp array.
  2. The VM is stopped on VMware, and the Microsoft conversion tool is used with the NetApp VMDK to VHD conversion cmdlet to quickly convert to a Hyper-V format VM with VHDX.
  3. The VM is started on Hyper-V.
  4. The VM is migrated using Live Storage Move to the final storage.

This enables the VM to be down for a very small amount of time as the actual migration between storage is done while the VM is running. The only downtime required is that needed to perform the conversion. There is a nice video showing this in action.