Disk Merge Issue for the Restore Point (Code 1954)

Situation

After a Hyper-V virtual machine backup completes, the temporary checkpoint created for the backup is removed. However, the Hyper-V host fails to merge the differential .avhdx files back into the parent .vhdx files.

A backup plan run finishes with the following info message: *Disk merge issue for the restore point. The Hyper-V host fails to merge .avhdx files due to issues on the Hyper-V side.

The backup itself is successfully completed and valid. The backup data was read from the checkpoint before it was removed.

The issue affects the state of the virtual machine after the backup. Until the merge is completed, the VM continues to run using the .avhdx chain.

Cause

The merge is performed by the Hyper-V host, not by the backup application. A merge may fail or not start because of issues on the Hyper-V side, for example:

• Insufficient free space on the volume containing the virtual machine disks. A merge requires additional free space.
• An antivirus or other filter driver is holding one of the virtual disk files.
• Known Hyper-V or VMMS issues that can be resolved by installing the recommended updates or hotfixes.
• The VM is in a state that prevents a live merge.
• Problems with the parent-child relationships in the virtual disk chain.

Impact

The backup remains valid and can be used for restore.

However, if the merge does not complete, the VM continues to use the .avhdx files. If subsequent backups create additional checkpoints before the existing chain is merged, the chain can become longer.

A long .avhdx chain can:

• Reduce the virtual machine's disk performance.
• Consume additional disk space on the Hyper-V host.
• Increase the risk of problems if the checkpoint chain is manually modified or deleted.

Solution

Do not manually delete .avhdx files. They are part of the VM's active virtual disk chain.

Try the following steps:

• Update Backup for Windows on the affected machine to the latest available version.
• Install the recommended Hyper-V updates and hotfixes on the Hyper-V server.
• Restart the Hyper-V host before the next backup.

Also make sure that the volume containing the VM's virtual disks has sufficient free space and that antivirus or other security software is not interfering with the virtual disk files.

If the issue persists after these steps, check the Hyper-V and VMMS event logs for errors related to checkpoint or virtual disk merge operations.

When to investigate

This warning is usually self-resolving if it occurs only once.

Hyper-V performs the merge in the background, so the merge may complete several minutes after the backup finishes or after the VMMS service is restarted.

Before investigating the issue, check whether the .avhdx files have already been merged and removed automatically.

Investigate the issue if the warning repeats during subsequent backup runs and the .avhdx chain continues to grow. A growing chain may affect VM disk performance and consume additional disk space on the Hyper-V host.