Hyper-V wait for merge

SUMMARY

After backing up a Hyper-V VM, unexpected avhd(x) files are listed as backed up files

ISSUE

If the VM has user created checkpoints, then seeing avhd(x) files (other than AutoRecovery.axhd(x)) is normal. However, if the customer has not created checkpoints, then the extra avhd(x) files are most likely in the backup list because a new backup was started before the Microsoft Hypervisor completed its merge process. Once the merge process completes, the extra avhd(x) file, which are part of the hypervisor's backup events, will be removed.

RESOLUTION

A new configuration parameter has been added to the Unitrends Agent MASTER.INI file. This parameter tells the Unitrends agent to wait for the Hypervisor merge process to complete, before starting the next VM backup. When the backup request for a given VM is received by the agent, if that VM is processing a merge, the agent will wait until either the merge completes or the configured time has expired. A VM backup will only wait if the same VM is currently waiting on a merge to complete. A given VM will never wait if a different VM is processing a merge. If the previous merge process for that VM has already completed, the backup will not wait at all. If a merge process is currently running, then the MASTER.INI entry value will specify how many seconds the backup should wait before aborting. The new MASTER.INI entry is found in the [Hyper-V] section. The new entry name is WaitForMerge. When it is set to 0, this feature is disabled. When set to a value greater than 0, it represents how many seconds the agent will wait before aborting the backup. If the merge completes before the wait time expires, the backup will continue as normal. The following entry would wait for 5 minutes.

[Hyper-V]
WaitForMerge=300

CAUSE

During a VM backup, the Microsoft Hypervisor creates a temporary vhd(x) file. When the backup completes, the Hypervisor must merge the data from the avhd(x) file with the original vhd(x) file. The time to complete the merge process depends on how large the avhd(x) file is. During a VM backup, the more the VM data is modified, the larger the avhd(x) file becomes, and the longer the merge process takes. If a VM is processing a merge and another backup is started for that same VM, the Hypervisor will include the temporary avhd(x) files in the list of files to back up. However, if the merge were completed first, the axhd(x) files would not be included in files that are backed up.

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