Merging a 140GB Hyper-V 2008 R2 snapshot

Last week I was notified that one of the production LUNs of a customer using Hyper-V 2008 R2 was filling up and the reason for this was a ‘deleted’ snapshot of a production system.

Deleting snapshots in Hyper-V 2008 R2 requires a shutdown of the VM in order to completely remove the snapshot (AVHD file) on your storage system. Just removing the snapshot/checkpoint using the Virtual Machine Manager is not sufficient. The AVHD file will still exist and keeps growing until you shut the VM down. This is a feature according to Microsoft.

This growing had been going on for a few weeks and the AVHD file has reached a size of 140GB. We made a rough estimation that the storage system would support a minimum of 15 MB/s throughput and with the size we had to process, this would’ve taken 2 to 3 hours. That meant 2 to 3 hours downtime for this particular VM.

Some people on the net were arguing whether extra space was required on the Cluster Shared Volume to merge the snapshot. This is not true.

Just to be sure, I created a backup of the VM just before starting the merge / shutting down the VM. After office hours I shut down the VM and kept an eye on the merge progress using the following PowerShell command:

Get-WmiObject -Namespace "rootvirtualization" -Query "select * from Msvm_ConcreteJob" | Where {$_.ElementName -eq 'Merge in Progress'}

The merge started within 5 minutes after shutting the VM down and within 15 minutes it reached about 5 percent. In just 90 minutes the merge was completed and the VM was booted back up to restore functionality!

So, snapshotting in Hyper-V 2008 R2 is still shit. It still requires downtime but not as much as calculated. This ‘feature’ is removed in Hyper-V 2012 and will automatically clean up after itself 🙂

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