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Don't mount your Hyper-V disk and change the contents if it has checkpoints

Posted by Dominic Cronin at Oct 16, 2020 05:54 PM |
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I just managed to get myself into trouble with Hyper-V. I'm busy setting up a new Tridion image, so I'd started with a fresh Windows Server Essentials install, and then once I had that running, I wanted to copy a handful of installers from the host computer to the new image. What could be simpler than just mounting the VHDX, I thought. Wrong!

So.... mounting the virtual disk is easy, you just right-click on the file, and Windows offers you a Mount option in the context menu. You have to stop the virtual server first, but OK. So I did this - copied my installers over, unmounted the drive with the Eject option from the context menu of what had become E: and went back to start the image again. This promptly failed with various messages that said there was a mismatch between the differencing virtual disk and the parent disk. I hadn't really wanted a differencing disk (which is what it turns out a checkpoint is really called). Checkpoint shmeckpoint... this is not a highly available super reliable server I'm building. Anyway  - checkpoints... it looks like it's just journaling; all your edits go in the checkpoint file, and I suppose a restore is just deleting the checkpoint file.

Enough speculation - what it means is that for the disk to work, Hyper-V has to know the order that the various slices are layered on top of each other, and when it does that - it also does some integrity checking. Not a bad thing, you might think, but adding some files had presumably borked a checksum or whatever, and it was throwing up the mismatch message. Apparently you can fix this through the Inspect button in the user interface, but then I got a different error. Fortunately - everything in Hyper-V works from the command line too, so from the powershell I was able to do the following and it was all good again.

Set-VHD .\some_checkpoint_or_other.avhdx -ParentPath .\TheVirtualDisk.vhdx -IgnoreIdMismatch

If you have more checkpoints, you can make each one in turn a parent of the other in the same way.

Mostly - this post is about the fact that it's apparently stupid to mount a VHDX and edit it, and nobody had told me. Next time I'll just run up a share.

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