Hyper-V: Dynamic virtual hard disks are not recommended for virtual machines that run server workloads in a production environment

Applies To: Windows Server 2008 R2

This topic is intended to address a specific issue identified by a Best Practices Analyzer scan. You should apply the information in this topic only to computers that have had the Hyper-V Best Practices Analyzer run against them and are experiencing the issue addressed by this topic. For more information about best practices and scans, see Best Practices Analyzer.

Issue

A virtual machine is configured with one or more dynamically expanding virtual hard disks (VHDs).

Impact

Available space may run out on the physical disk that stores the .vhd files. If this occurs, no additional disk operations can be performed on the physical storage. Any virtual machine that relies on the physical storage could be affected.

If physical disk space runs out, any running virtual machine that has snapshots or virtual hard disks stored on that disk may be paused automatically. Hyper-V Manager shows the status of these virtual machines as “paused-critical”.

Resolution

Shut down the virtual machine. Use the Edit Disk Wizard in Hyper-V Manager to convert each dynamically expanding virtual hard disk for this virtual machine to a fixed sized virtual hard disk.

Before you perform the following procedure, review the following considerations to help avoid problems.

• Make sure you have enough free physical storage space to store the .vhd files of the converted disks. The .vhd file of each fixed virtual hard disk requires the same amount of space as the maximum size specified for the virtual hard disk when it was created. (In Hyper-V Manager, under Actions, click Inspect Disk to check the maximum size.)

• Merge or delete all snapshots before you convert the disk. Failure to do so will make any remaining snapshots unusable.

• The dynamically expanding virtual hard disk must not be used as a parent of a differencing disk. Converting a parent disk will render it unusable by all differencing disks associated with it.

By default, membership in the local Administrators group, or equivalent, is the minimum required to complete this procedure.

To convert a virtual hard disk

1. Open Hyper-V Manager. Click Start, point to Administrative Tools, and then click Hyper-V Manager.

2. If the virtual machine is running, shut it down. Right-click the name of the virtual machine, and then click Shut Down.

3. From the Actions menu, click Edit Disk.

4. On the Locate Virtual Hard Disk page, type or browse to the location of the fixed virtual hard disk.

5. On the Choose Action page, click Convert.

6. On the Convert Virtual Hard Disk page, specify a name and location for the converted disk.

7. On the Summary page, review the details and then click Finish.

Additional references

For more information about settings and configurations that can affect server performance, see Performance Tuning Guidelines for Windows Server 2008 R2 (https://go.microsoft.com/fwlink/?LinkId=190194).