Differencing Disk Files (avhdx) Are Not Merged After Checkpoint Is Created

Situation

When performing Hyper-V backups, Backup for Windows addresses a Hyper-V server to create a checkpoint. As a part of created checkpoint *.avhdx differencing disk files are created as children of virtual machine disk/checkpoint differencing disks.

As the Hyper-V backup plan completes, the checkpoint must be removed, and differencing *.avhdx files must be merged with parent disk files.

In some cases, the Hyper-V server does not merge them. This results in an impressive amount of auxiliary *.avhdx files left in Hyper-V hard disks locations.

In case the backup plan is configured to back up a checkpoint tree, this may lead to a significant backup size and backup time, respectively.

Cause

Most probably, VM disk merge fails due to disk is captured by third-party software.

Solutions

Since the cause of such Hyper-V server behavior is not clear, there may be several solutions for this case. Run across the possible solutions to find the right one.

Upgrade Backup for Windows to the Latest Version

  1. In the application menu, select Help, then select Check for Updates.
  2. If a new version is found, download and install it.
  3. Run the Hyper-V backup plan again.

Install Recommended Hotfixes for Hyper-V Servers

Install Hyper-V server hotfixes according to your operating system version.

Windows 2012 R2

  • http://support.microsoft.com/kb/3090343. Resolves an issue that occurs when you use the Volume Shadow Copy Service (VSS) backup on a Cluster Shared Volumes (CSV) volume by using a software snapshot provider in Windows Server 2012 R2
  • http://support.microsoft.com/kb/3072380. Resolves an issue on a Hyper-V cluster where the heartbeat component times out during a long-running snapshot. The snapshot fails and the VM restarts. Available for individual download. To apply this update, you must first install the update 2919355 on Windows Server 2012 R2
  • http://support.microsoft.com/kb/3068445. Resolves an issue where a deadlock or Stop 0x9E occurs on a cluster node when there is a lot of network (SMB) I/O requests to large files (1 TB or more). Includes the fix from 3044457. Available for individual download. To apply this update, you must first install the update 2919355 on Windows Server 2012 R2
  • http://support.microsoft.com/kb/3068444. Resolves an issue where a cluster node experiences a Stop 0xF5 FLTMGR_FILE_SYSTEM when using Shared VHDX. Available for individual download. To apply this update, you must first install the update 2919355 on Windows Server 2012 R2
  • http://support.microsoft.com/kb/2919355. Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 Update April 2014 Multiple files ​A cumulative update that includes the security updates and nonsecurity updates including Failover Clustering updates that were released before March 2014. Available from Windows Update and for individual download from Download Center
  • http://support.microsoft.com/kb/976424. Error code when the kpasswd protocol fails after you perform an authoritative restore: "KDC_ERROR_S_PRINCIPAL_UNKNOWN" KDCSVC Enables you to add a Windows Server 2012 failover cluster. Install on every domain controller that is running Windows Server 2008 Service Pack 2 (SP2) or Windows Server 2008 R2. Otherwise, Create Cluster may fail when you try to set the password for the cluster computer object, and you receive a "CreateClusterNameCOIfNotExists (6783): Unable to set a password on <ClusterName$>" error message. This hotfix is included in Windows Server 2008 R2 Service Pack 1 (SP1)

Windows 2012

Windows 2008 R2 SP1

  • http://support.microsoft.com/kb/2264080. An update rollup package for the Hyper-V role in Windows Server 2008 R2: August 24, 2010
  • http://support.microsoft.com/kb/975695. When you try to back up a virtual machine on a Windows Server 2008-based computer that has multiple virtual hard disks attached, the backup fails, and Hyper-V Writer error "0x800423f4" is generated
  • http://support.microsoft.com/kb/2223005. The network connection is lost on a Hyper-V virtual machine
  • http://support.microsoft.com/kb/976424. Error code when the kpasswd protocol fails after you perform an authoritative restore: "KDC_ERROR_S_PRINCIPAL_UNKNOWN" KDCSVC Install on every domain controller that is running Windows Server 2008 Service Pack 2 or Windows Server 2008 R2 in order to add a Windows Server 2012 failover cluster. Otherwise, Create Cluster may fail with an error message: CreateClusterNameCOIfNotExists (6783): Unable to set password on <ClusterName$> when it tries to set the password for the cluster computer object. This hotfix is included in Windows Server 2008 R2 Service Pack 1.

Virtual Machine Start/Restart

To restart VM on the Hyper-V server, proceed as follows:

  1. Open Hyper-V Manager.
  2. In the Virtual Machines window, select the required virtual machine, then right-click on it.
  3. In the context menu, select Reset.
  4. Repeat steps 2-3 for all running virtual machines.

Hyper-V Server Reboot

If a VM import does not help and merge is still not performed, restart the Hyper-V server.

Hyper-V Machine Import

To import virtual machines, proceed as follows:

  1. Open Hyper-V Manager.
  2. In the Actions menu, click Import Virtual Machine.
  3. Click Next.
  4. Select the folder that contains the exported files, and click Next.
  5. Select the virtual machine to import.
  6. Choose the import type, and click Next.
  7. Click Finish.

To learn more about virtual machine import, refer to the Export and Import virtual machines chapter at docs.microsoft.com