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I have a Windows 2012 R2 Hyper-V server that has two virtual machines running.

I have Windows backup configured to do a full bare metal backup of the Hyper-V server to USB disk.

Every few weeks the virtual machines become paused overnight. On investigation this is due to lack of disk space. On the GUI the checkpoints panel for both VMs is empty. However the Virtual Disks folder contains many .avhdx files with the exact time stamp of the backup start time.

It seems that Windows Backup is not cleaning up the checkpoint files after its backup. These files are not available in the GUI or by running the powershell command Get-VMSnapshot -ComputerName "vm01" -VMName "server"

The only way to clear these checkpoints is to shut down the virtual machine and wait for the merge to complete.

Have I missed something or is this a limitation of Hyper-V?

  • 1) what are the guest machines OSs? 2) have you tried a full Windows Update on hosts and guests? most of the backup stuff will be solved by updating 3) any errors in the event log that you can share? – Noor Khaldi Oct 31 '16 at 20:53
  • The guest OSs are SBS 2011 and Windows Server 2008 R2. Both VMs are fully up to date with Windows updates and so is the host. – alcomcomputing Nov 1 '16 at 8:31
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Take note of this note from MSDN:

Backing Up the Virtual Machines Hyper-V uses one of two mechanisms to back up each VM. The default backup mechanism is called the "Saved State" method, where the VM is put into a saved state during the processing of the PrepareForSnapshot event, snapshots are taken of the appropriate volumes, and the VM is returned to the previous state during the processing of the PostSnapshot event. The other backup mechanism is called the "Child VM Snapshot" method, which uses VSS inside the child VM to participate in the backup.).

Assuming that you are trying to use the "child VM mechanism", have you made sure you meet the critera outlined?

  • Backup (volume snapshot) Integration Service is installed and running in the child VM. The service name is "Hyper-V Volume Shadow Copy Requestor". Windows 2000: Backup Integration Service is not supported.
  • The child VM must be in the running state.
  • The Snapshot File Location for the VM is set to be the same volume in the host operating system as the VHD files for the VM.
  • All volumes in the child VM are basic disks and there are no dynamic disks.
  • All disks in the child VM must use a file system that supports snapshots (for example, NTFS).

If all of those critera are met, I would review the the following:

1) Are the VSS providers / writers in a healthy state on both the host and the guest VMs? This article from Veritas will show you how to check. It's essentially these two commands.

vssadmin list writers
vssadmin list providers

2) Are the integration services up to date on the guest VMs? Try to install the integration services, if it is out of date you will be prompted to upgrade.

3) Have you reviewed the Windows backup logs to determine why this is occuring? The default log location is %windir%\logs\windowsserverbackup - additonal logs are outlined in this Technet post.

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