Hyper-V Backup fails with Disk Merge error HYPERV23

Problem description

Hyper-V Virtual Machine backups fail with error HYPERV23. This occurs when a backup process encounters lingering or stale checkpoints (.avhd / .avhdx files) on the Hyper-V host that cannot be automatically merged by the host during the post-data upload phase.

Cause

An existing VM checkpoint was not merged automatically by the Hyper-V host, preventing the Druva inSync Agent from completing the pending disk merge job required during backup operations.

Traceback

[2021-11-20 00:09:45,256] [ERROR] <_MainThread(MainThread)> Error <class 'inSyncLib.inSyncError.SyncError'>:Disk Merge of <Virtual machine Name> Failed: (#100090017) (Error Code : HYPERV23). Traceback -Traceback (most recent call last): File "agentPartnerProcess.pyc", line 157, in _process_request File "agentPartnerProcess.pyc", line 85, in _process_request_start File "agents\hyperv\hypervAgent.pyc", line 70, in process_backup_start File "agents\hyperv\hypervBackup.pyc", line 810, in backup File "agents\hyperv\rctHelper.pyc", line 97, in post_data_upload File "agents\hyperv\hypervapi\hypervMachine.pyc", line 522, in wait_for_pending_disk_merge_job File "inSyncLib\inSyncError.pyc", line 30, in raise_error SyncError: Disk Merge of VM <Virtual Machine Name> Failed: (#100090017) (Error Code : HYPERV23)

Resolution

Step 1: Delete Checkpoint via Hyper-V Manager

Open Hyper-V Manager on the host machine.
Select the affected Virtual Machine and verify if lingering checkpoints exist.
If a checkpoint exists, right-click the checkpoint and select Delete Checkpoint Subtree or Delete Checkpoint.
Power off the Virtual Machine.
Trigger a full or differential backup from the Druva Management Console.
Power on the Virtual Machine and verify that subsequent scheduled backups succeed.

Step 2: Manually Merge Disks (If automated merge fails)

Power off the affected Virtual Machine.
Open Hyper-V Manager, select Edit Disk, and inspect the chain of .avhd / .avhdx differential files.
Manually merge the differential checkpoint file into its parent .vhd / .vhdx base disk using Hyper-V Manager or PowerShell.
Update the VM hardware settings to point directly to the merged parent VHD/VHDX file.
Power on the VM and run an on-demand backup.

Verification

Navigate to the Druva Management Console and initiate an on-demand backup for the affected Hyper-V Virtual Machine.
Ensure the backup job completes with status Successful.
Verify in Hyper-V Manager that post-backup temporary checkpoints are cleanly removed without leaving orphaned .avhd or .avhdx files.