What happens if I start a virtual machine that is merging snapshot files? [Hyper-V]

What happens if I start a virtual machine that is merging snapshot files? [Hyper-V]

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If you have a virtual machine with snapshots that have been deleted, it might decide to start merging in the snapshot AVHD files the next time the virtual machine turns off.  But what if you were only planning to turn off the virtual machine for 5 minutes while you made some configuration changes?

Well, the good news is that you can still start up the virtual machine.  Starting the virtual machine will cancel the merge operation and the virtual machine will start and function correctly.  The bad news, however, is that when you start the virtual machine all progress made on the merge operation will be thrown away.  The next time the virtual machine shuts down the merge will start over again – from the beginning.  This means that in order for the snapshot files to merge in successfully you need to ensure that the virtual machine is offline for the entire amount of time for the merge operation to complete.


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  • So, if I shut down a virtual machine, how long should I expect it to take before the disks get merged? I have a virt hd that is approx. 150GB, and a snapshot I accidently created, so the differencing disk is 67GB! I have deleted the snapshot, but I haven't had the VM down long enough for the merge to happen.

    I understand there are lots of variables (speed of disk subsystem, etc.), but is there any way to monitor the progress of the merge? Is the only way to know it's complete is to check the differencing disk to see when it's gone?

    Also, is there no way to live-merge?

    Thanks for the tips...very insightful! Please keep them coming.


  • Cheers, was hoping that it wouldnt cause any problems but now I know.

    Cheers for the excellent blog.

  • I've noticed that you don't have to start the VM to cancel the merge - simply editing the VM (I increased the memory from 3gig to 4gig) will also do so.

    Worse, it sets the JobStatus of the Msvm_ConcreteJob to "{Job is completed, Job is completed successfully}" which then leads you to assume that the job is completed and completed successfully.  The only clue that Hyper-V is telling lies is that the PercentComplete property is less than 100.

    (I had to power-up and immediately switch-off the VM to re-trigger the merge again)

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