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How does the shared VHDX file feature in Windows Server 2012 R2 work?

The new shared VHDX feature brings shared storage to guest clusters by eliminating the need for an iSCSI or Fibre Channel connection.

Can you explain the shared VHDX file feature in Windows Server 2012 R2?

Brien PoseyBrien Posey

One of the most promising new storage-related features in Windows Server 2012 R2 Hyper-V is shared VHDX. The basic idea behind this feature is that it makes it possible to create a virtual hard disk file and share it among multiple virtual machines (VMs).

A shared VHDX file is intended for use with guest clustering. Previously, if you wanted to use shared storage for a guest cluster, the cluster nodes had to connect to a cluster shared volume by way of iSCSI, Fibre Channel or a Server Message Block (SMB) 3.0 file share (Hyper-V 3.0 only). In Windows Server 2012 R2, it will be possible to share a virtual hard disk (VHDX file format only) so that it appears to guest cluster nodes as if it were a shared Serial Attached SCSI disk.

So, why will shared VHDX be useful? Many larger organizations have constructed private clouds in which authorized users are allowed to create their own VMs that are based on pre-approved templates. Self-service virtual machine creation works great for building standalone VMs, but it can be problematic for building guest clusters.

Although there are exceptions, guest clusters typically require the cluster nodes to access Cluster Shared Volumes (CSV). However, virtualization administrators are understandably reluctant to expose the underlying storage architecture in a way that allows users to create and manage a CSV.

This is where shared VHDX comes into play. Since administrators generally don't want to expose the storage infrastructure, they can instead allow users to create a VHDX file, and share that file among their guest cluster nodes as if the shared VHDX file were a true CSV.

It is worth noting that shared VHDX files do not eliminate the need for physical shared storage. The shared VHDX file must reside in a location where it is accessible to the guest cluster nodes. This means it must reside on a CSV or on an SMB 3.0 file share. However, it is possible for virtualization administrators to allow for the creation of a shared VHDX without having to completely expose the underlying storage architecture.

In addition to being useful in self-service environments, the shared VHDX feature will be useful in hybrid clouds because it will reduce the complexity of connecting cloud-based guest cluster nodes to shared storage.

About the author:
Brien Posey is a Microsoft MVP with two decades of IT experience. Before becoming a freelance technical writer, Brien worked as a CIO for a national chain of hospitals and healthcare facilities. He has also served as a network administrator for some of the nation's largest insurance companies and for the Department of Defense at Fort Knox.

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