Getting to know VHDX: The new Hyper-V virtual hard disk format

The VHDX format in Windows Server 2012 has the decisive edge over the legacy VHD format in almost every respect, including performance and capacity.

Windows Server 2012 Hyper-V will feature the new VHDX virtual hard disk format. In many ways, VHDX disks will offer superior performance, capacity and reliability over Hyper-V’s legacy Virtual Hard Disks.

In addition, Microsoft will provide an easy way for IT shops to convert Virtual Hard Disk (VHD) files to the VHDX format, after upgrading to Windows Server 2012 Hyper-V.

Specifics on VHDX capacity limits and compatibility

With the VHDX format, Hyper-V administrators will be able to create much larger virtual hard disks. Hyper-V currently has a 2 TB limit for VHDs, but VHDX files will have a 64 TB capacity.

Windows Server 2012 supports VHD and VHDX virtual hard disks
Figure 1: Windows Server 2012 supports VHD and VHDX virtual hard disks.

Windows Server 2012 is still in beta testing, so Microsoft could make changes prior to the final release. As it stands, Windows Server 2012 allows you to create VHDX as well as legacy-format VHDs, but Windows Server 2008 will support only VHD files.

Fixed, dynamically expanding or differencing disks

The upcoming version of Windows Server will still offer fixed and dynamically expanding virtual hard disks; however, Microsoft also added the ability to create differencing disks in the Hyper-V Manager, shown in Figure 2.

Differencing disks are not new to Hyper-V. When you create a virtual machine (VM) snapshot in Hyper-V,  you’re actually creating a differencing disk. All write operations are redirected to the differencing disk so that the original virtual hard disk (now a snapshot) remains unmodified.

Microsoft has added the option to create a differencing disk
Figure 2: Microsoft has added the option to create a differencing disk.

Microsoft has presumably added the option to create differencing disks to make it easier to build virtual machines from a VM template. This approach allows you to spawn multiple virtual machines from a parent VM.

The VHDX file format also offers benefits beyond higher capacity. VHDX files support larger block sizes for dynamically expanding and differencing disks, with a logical sector size of 4 KB, leading to better performance over legacy VHD files. Additionally, Microsoft has taken steps to prevent corruption within VHDX files in the event of a power failure (e.g., updates are logged to the VHDX metadata as a fallback mechanism).

VHD to VHDX: The conversion process

IT pros with existing Hyper-V environments may wonder if their workloads can benefit from the new VHDX features. Fortunately, once organizations move to Windows Server 2012, they can convert VHD files to the VHDX format.

The conversion process is relatively simple. When you right-click on a host server within the Hyper-V Manager, a sub menu appears, which contains an Edit Disk option. Choosing that option opens a wizard, which prompts you to select a VHD. The next screen will present you with an option to convert the VHD.  Then, you are given the option to convert the VHD file to VHDX format, as shown in Figure 3.

You can convert a VHD file to VHDX
Figure 3: You can convert a VHD file to VHDX.

As you prepare to convert legacy VHDs to the VHDX format, keep in mind that the virtual hard disk will be unavailable during the conversion process. It will create a new virtual hard disk file and delete the old one. As such, you will need enough physical disk space to temporarily accommodate two copies of the virtual hard disk that you are converting.

As you can see, the new VHDX format offers a number of benefits over traditional VHDs. To experience those benefits with existing virtual machines, however, you must first migrate the VMs to Windows Server 2012 Hyper-V servers.

Next Steps

Managing Virtual Hard Disk data with encapsulation

Mounting VHD files in Hyper-V environments

This was first published in June 2012

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