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Advantages to the Hyper-V VHDX format

The VHDX file format offers new features and better scalability over its predecessor, but many IT professionals still have questions about the advantages.

The new Hyper-V VHDX format is one of the most important features available in Windows Server 2012 and 2012 R2. A virtual disk file captures the state of the VM in a server's memory and represents that state as a file. This new virtual hard disk format increased virtual disk capacity and opened the door to several other important features. However, it's also one of the most common features that IT pros have questions about. If you want to know more about the benefits of the VHDX format or are unsure whether to use the VHD or newer VHDX format, here are some answers to frequently asked questions that can help guide your decisions.

Should I use the Hyper-V VHDX format rather than the VHD format?

The short answer is yes. There are many advantages to using the new file format. However, this answer assumes you are using Windows Server 2012 or 2012 R2. The VHDX format is not compatible with earlier versions of Windows Server. So, if you expect you may need to migrate your VMs to a host running an earlier version of the operating system, you will likely want to stick with the legacy VHD format.

How does the VHDX offer better scalability and performance?

For starters, Microsoft significantly increased the virtual disk capacity, from 2 TB available for the VHD format to 64 TB for the Hyper-V VHDX format. This increase alone allows for the large VMs that are becoming more common in modern virtual data centers. There are other important scalability changes, including increases in the logical sector size from 512 bytes in the VHD format to 4 KB in VHDX, which improves disk I/O performance. The new VHDX format also protects against file corruption that can result from unexpected power failures by tracking changes in metadata.

How does a shared VHDX work?

The shared virtual hard disk feature is available only with the newer VHDX file format and with Windows Server 2012 R2. A shared virtual hard disk allows VMs on the same guest cluster to share a VHDX file that can be used to host clustered resources, such as SQL Server database files. This allows simpler guest failover clusters that can protect applications running in VMs.

Does a Generation 2 VM need to use the VHDX format?

Yes. A Generation 2 VM can run more efficiently than its first generation counterpart because the VMs are hypervisor aware and do not rely on emulated hardware, but require the newer VHDX file format. However, Microsoft has eased the conversion process, allowing users to convert Generation 1 VM data from a VHD file to a VHDX file to use in Generation 2 VMs.

How can I resize a VHDX file?

With the legacy VHD format, you needed to shut down the VM to change the size of the VM's virtual hard disk. Resizing virtual hard disks is easier with the VHDX file format, in fact, you can resize the hard disk size without end users even realizing the change. You can resize a VHDX either through the Edit Virtual Hard Disk Wizard found in Hyper-V Manager or with PowerShell, using a basic command called Resize-VHD.

Next Steps

Easing your VM migration

This was last published in November 2014

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The 64 TB limit on VHDX will be needed for those large SQL server databases and they can be shared over a cluster of servers.
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