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Hyper-V: Avoid using differencing disks on 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.

Operating System

Windows Server 2008 R2








A virtual machine is configured with one or more differencing virtual hard disks.


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”.


Shut down the virtual machine. In Hyper-V Manager, inspect the differencing disk to determine the parent virtual hard disk. If you merge a differencing disk to a parent disk that is shared by other differencing disks, that action will corrupt the relationship between the other differencing disks and the parent disk, making them unusable. After verifying that the parent virtual hard disk is not shared, you can use the Edit Disk Wizard to merge the differencing disk to the parent virtual hard disk.

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


Before you perform the following procedure, make sure that the differencing disk or the parent disk is not associated with any other differencing disks. Modifying a virtual hard disk that is associated with another virtual hard disk will render the other disk unusable.

To merge the differencing disk to the parent virtual hard disk

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

  2. Shut down the virtual machine. 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 differencing disk.

  5. On the Choose Action page, click Merge.

  6. On the Merge Changes from Differencing Disk page, leave the option set as To the parent virtual hard disk.

  7. Click Finish to complete the wizard, or click Next to review the changes on the Summary page and then click Finish.

After you merge the differencing disk, the virtual machine must be reconfigured to use the parent disk instead of the differencing disk. To do this, you attach the parent disk to the virtual machine.

To attach the parent disk to the virtual machine

  1. From the details pane of Hyper-V Manager, right-click the virtual machine and then click Settings.

  2. In the navigation pane, find the IDE controller or SCSI controller that the differencing disk was attached to. Below the controller name, click Hard Drive.

  3. On the Hard Drive page, under Media, the path and file name of the differencing disk are listed. Click Browse.

  4. Select the parent disk, and then click Open.

  5. On the Hard Drive page, under Media, the path and file name of the parent disk are now listed. Click OK.

  6. If you want to recover the space used by the differencing disk, you can delete it now without having any effect on the virtual machine.