Shared VHDX In Windows 2016: VHDS and the backing storage file

Introduction into the VHD Set

I have talked about the VHD Set with a VHDS file and a AVHDX backing storage file in Windows Server 2016 in a previous blog post A first look at shared virtual disks in Windows Server 2016. One of the questions I saw pass by a couple of times is whether this is still a “normal VHDX” or a new type of virtual disk. Well the VHDS files is northing but a small file containing some metadata to coordinate disk actions amongst the guest cluster nodes accessing the shared virtual disk. The avhdx file associated with that VHDS file is an automatically managed dynamically expanding or fixed virtual disk. How do I know this? Well I tested it.

There is nothing that preventing you from copying or moving the avhdx file of a VHD Set that not in use. You can rename the extension from avhdx to vhdx. You can attach it to another VM or mount it in the host and get to the data. In essence this is a vhdx file. The “a” in avhdx stands for automatic. The meaning of this is that an vhdx is under control of the hypervisor and you’re not supposed to be manipulating it but let the hypervisor handle this for you. But as you can see for yourself if you try the above you can get to the data if that’s the only option left. Normally you should just leave it alone. It does however serve as proof that the VHD Set uses an standard virtuak disk (VHDX) file.

I’ll demonstrate this with an example below.

Fun with a backing storage file in a VHD Set

Shut down all the nodes of the guest cluster so that the VHD Set files are not in use. We then rename the virtual disk’s extension avhdx to vhdx.

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You can then mount it on the host.

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And after mounting the VHDX we can see the content of the virtual disk we put there when it was a CSV in that guest cluster.

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We add some files while this vhdx is mounted on the host

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Rename the virtual disk back to a avhdx extension.

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We boot the nodes of the guest cluster and have a look at the data on the CSV. Bingo!

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I’m NOT advocating you do this as a standard operation procedure. This is a demo to show you that the backing storage files are normal VHDX files that are managed by the hypervisor and as such get the avhdx extension (automatic vhdx) to indicate that you should not manipulate it under normal circumstances. But in a pinch, it a normal virtual disk so you can get to it with all options and tools at your disposal if needed.

Recover From Expanding VHD or VDHX Files On VMs With Checkpoints

So you’ve expanded the virtual disk (VHD/VHDX) of a virtual machine that has checkpoints (or snapshots as they used to be called) on it. Did you forget about them?  Did you really leave them lingering around for that long?  Bad practice and not supported (we don’t have production snapshots yet, that’s for Windows Server 2016). Anyway your virtual machine won’t boot. Depending on the importance of that VM you might be chewed out big time or ridiculed. But what if you don’t have a restore that works? Suddenly it’s might have become a resume generating event.

All does not have to be lost. Their might be hope if you didn’t panic and made even more bad decisions. Please, if you’re unsure what to do, call an expert, a real one, or at least some one who knows real experts. It also helps if you have spare disk space, the fast sort if possible and a Hyper-V node where you can work without risk. We’ll walk you through the scenarios for both a VHDX and a VHD.

How did you get into this pickle?

If you go to the Edit Virtual Hard Disk Wizard via the VM settings it won’t allow for that if the VM has checkpoints, whether the VM is online or not.

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VHDs cannot be expanded on line. If the VM had checkpoints it must have been shut down when you expanded the VHD. If you went to the Edit Disk tool in Hyper-V Manager directly to open up the disk you don’t get a warning. It’s treated as a virtual disk that’s not in use. Same deal if you do it in PowerShell

Resize-VHD -Path “C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD.vhd” -SizeBytes 15GB

That just works.

VHDXs can be expanded on online if they’re attached to a vSCSI controller. But if the VM has checkpoints it will not allow for expanding.

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So yes, you deliberately shut it down to be able to do it with the the Edit Disk tool in Hyper-V Manager. I know, the warning message was not specific enough but consider this. The Edit disk tool when launched directly has no idea of what the disk you’re opening is used for, only if it’s online / locked.

Anyway the result is the same for the VM whether it was a VHD or a VHDX. An error when you start it up.

[Window Title]
Hyper-V Manager

[Main Instruction]
An error occurred while attempting to start the selected virtual machine(s).

[Content]
‘DidierTest06’ failed to start.

Synthetic SCSI Controller (Instance ID 92ABA591-75A7-47B3-A078-050E757B769A): Failed to Power on with Error ‘The chain of virtual hard disks is corrupted. There is a mismatch in the virtual sizes of the parent virtual hard disk and differencing disk.’.

Virtual disk ‘C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD_8DFF476F-7A41-4E4D-B41F-C639478E3537.avhd’ failed to open because a problem occurred when attempting to open a virtual disk in the differencing chain, ‘C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD.vhd’: ‘The size of the virtual hard disk is not valid.’.

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You might want to delete the checkpoint but the merge will only succeed for the virtual disk that have not been expanded.  You actually don’t need to do this now, it’s better if you don’t, it saves you some stress and extra work. You could remove the expanded virtual disks from the VM. It will boot but in many cased the missing data on those disks are very bad news. But al least you’ve proven the root cause of your problems.

If you inspect the AVVHD/AVHDX file you’ll get an error that states

The differencing virtual disk chain is broken. Please reconnect the child to the correct parent virtual hard disk.

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However attempting to do so will fail in this case.

Failed to set new parent for the virtual disk.

The Hyper-V Virtual Machine Management service encountered an unexpected error: The chain of virtual hard disks is corrupted. There is a mismatch in the virtual sizes of the parent virtual hard disk and differencing disk. (0xC03A0017).

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Is there a fix?

Let’s say you don’t have a backup (shame on you). So now what? Make copies of the VHDX/AVHDX or VHD/AVHD and save guard those. You can also work on copies or on the original files.I’ll just the originals as this blog post is already way too long. If you. Note that some extra disk space and speed come in very handy now. You might even copy them of to a lab server. Takes more time but at least you’re not working on a production host than.

Working on the original virtual disk files (VHD/AVHD and / or VHDX/AVHDX)

If you know the original size of the VHDX before you expanded it you can shrink it to exactly that. If you don’t there’s PowerShell to the rescue if you want to find out the minimum size.

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But even better you can shrink it to it’s minimum size, it’s a parameter!

Resize-VHD -Path “C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD.vhd” -ToMinimumSize

Now you not home yet. If you restart the VM right now it will fail … with the following error:

‘DidierTest06’ failed to start. (Virtual machine ID 7A54E4DB-7CCB-42A6-8917-50A05354634F)

‘DidierTest06’ Synthetic SCSI Controller (Instance ID 92ABA591-75A7-47B3-A078-050E757B769A): Failed to Power on with Error ‘The chain of virtual hard disks is corrupted. There is a mismatch in the identifiers of the parent virtual hard disk and differencing disk.’ (0xC03A000E). (Virtual machine ID 7A54E4DB-7CCB-42A6-8917-50A05354634F)

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What you need to do is reconnect the AVHDX to it’s parent and choose to ignore the ID mismatch. You can do this via Edit Disk in Hyper-V Manager of in PowerShell. For more information on manually merging & repairing checkpoints see my blogs on this subject here. In this post I’ll just show the screenshots as walk through.

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Once that’s done you’re VHDX is good to go.

For a VHD you can’t shrink that with the inbox tools. There is however a free command line tool that can do that names VHDTool.exe. The original is hard to find on the web so here is the installer if you need it. You only need the executable, which is portable actually, don’t install this on a production server. It has a repair switch to deal with just this occurrence!

Here’s an example of my lab …

D:\SysAdmin>VhdTool.exe /repair “C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD.vhd” “C:\ClusterStorage\Volume2\DidierTest06\Virtual Hard Disks\RuinFixedVHD_8DFF476F-7A41-4E4D-B41F-C639478E3537.avhd”

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That’s it for the VHD …

You’re back in business!  All that’s left to do is get rid of the checkpoints. So you delete them. If you wanted to apply them an get rid of the delta, you could have just removed the disks, re-added the VHD/VHDX and be done with it actually. But in most of these scenarios you want to keep the delta as you most probably didn’t even realize you still had checkpoints around. Zero data loss Winking smile.

Conclusion

Save your self the stress, hassle and possibly expense of hiring an expert.  How? Please do not expand a VHD or VHDX of a virtual machine that has checkpoints. It will cause boot issues with the expanded virtual disk or disks! You will be in a stressful, painful pickle where you might not get out of if you make the wrong decisions and choices!

As a closing note, you must have have backups and restores that you have tested. Do not rely on your smarts and creativity or that others, let alone luck. Luck runs out. Otions run out. Even for the best and luckiest of us. VEEAM has save my proverbial behind a few times already.

Manually Merging Hyper-V Checkpoints

A Last ditch Effort

Fist of all you need to realize this might not work. It’s a last ditch effort. There is a reason why you have backups (with tested restores) and why you should monitor your environment for things that are not as they should be. Early intervention can pay off.

Also see blog post on a couple of more preferred actions.

If you have lost checkpoints, you have basically lost data and corruption/data inconsistencies are very much a possibility and reality. If the files have been copied and information about what file is the parent the dates/timestamps are what you have to go by. You might not know for sure if you have them all.

Setting up the demo

For demo purposes we take a test VM and ad files to indicate what checkpoint we’re at.

We start with ORGINAL.TXT on the desktop and we create a checkpoint, which we rename accordingly.

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We add a file called CHECK01.TXT and we create a checkpoint, which we rename accordingly.

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We add a file called CHECK02.TXT and we create a checkpoint, which we rename accordingly.

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We add a file called NOW.TXT no more checkpoints are taken.

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The file names represent the content you’d see disappear if you applied the checkpoint and we have reflected this in the name for the checkpoints.

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As we want to merge all the snapshots and and up with a usable VHDX we’ll work back from the most recent differencing disk until all is merged. As you can see this is a straight forward situation and I hope you’ll never be running having to deal with a vast collection of sub trees Smile.

Finding out what are the parents of avhdx files

In this demo it’s pretty obvious what snapshot exist and what avhdx files they represent. We’ve even shown you the single tree visualized in Hyper-V Manager. In reality bad things  have happened and you don’t see this information anymore. So you might have to find out yourself. This is done via inspect disk in Hyper-V manager. I you’re confused about what the parent is of (a)vhdx files this tool will help you find out or show you what the most recent one was.

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Sometimes the original files have been renamed or moved and that it will show you’re the last known valid parent.

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Manually Merging the checkpoints

Remember to make a copy of all files as a backup! Also make sure you have enough free diskspace … you need working space! You might need another shot at this. As we want to merge all the snapshots and and up with a usable VHDX we’ll work back from the most recent differencing disk until all is merged in the oldest one which is the vhdx. You can look at the last modified time stamps to find out the correct order in which to work. The most recent avdx is the one used in the virtual machine configuration file and locate the information for the virtual hard disk.

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The configuration file’s avhdx is the one containing the “NOW” running state of the VM.

Note: You might find some information that you need to rename the extension avhdx to vhdx (or avhd to vhd). The reason for this was that in Windows 2008 Hyper-V Manager did not show avhd files in the Edit virtual disk wizard. You can still do this and it will still works, but you do not need to. Ever since Windows Server 2008 R2 avhd (and with since Windows Server 2012 avhdx) files do show up in Hyper-V Managers Disk edit.

For some insights as to why the order is important read this blog by Ben Armstrong What happens when a snapshot is being merged? [Hyper-V]

WARNING: If you did not start with the most recent one and work your way down, which is the easiest and least confusing way all is not lost. But you will have to reconnect the first more recent (a)vhdx to one to it’s new parent. This is needed as by merging a snapshot out of order more recent one will have lost it’s will have lost it’s original parent.

Here’s how to do this: Select reconnect. This is the page you’ll get if you’d start edit disk wizard as all other option are unavailable due to the missing parent.

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The wizard will tell you what used to be the parent and allow you to select a new one. Make sure to tick the check box for Ignore ID mismatch or the reconnect will fail as you’re previous out of order merge has created a new a(vhdx). If your in this pickle by renaming (a)vhdx files or after a copy this isn’t needed by the way.

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Follow the wizard after that and when your done you can launch the edit disk wizard again and perform a merge. It’s paramount that you do not mix up orders when doing so that you reconnect to the parent this or you’ll end up in a right mess. There are many permutations, keep it simple!. Do it in order Smile. If you start having multiple checkpoint trees/subtrees things can get confusing very fast.

You might also have to reconnect if the checkpoints have lost their connection the what they know to be their last parent for other reasons. In that case you do this and when that’s done, you merge. Rinse and repeat. The below walk through assumes you have no reconnects to be done. If so it will tell you like in the example above.

Walk trough:

Open the Edit Disk Wizardimage

Select the most recent avhdx & click “Next”

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We choose to merge the avhdx

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In our case into its parent disk

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Verify the options are correct and click “Finish”

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Let the wizard complete

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That’s it. You’ve merged the most recent snapshot into it’s parent. That means that you have not lost the most recent state of the virtual machine as when it was running before you shut it down. This can be verified by mounting the now most recent avhdx and looking at the desktop for my user profile. You can see the NOW.txt text file is there!

OK, dismount the avhdx and now it’s rinse and repeat.

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You do this over an over again until your merge the last avhdx into the vhdx.

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Than you have the vhdx you will use to create a new virtual machine.

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Make sure you get the generation right.

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Assign memory

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Connect to the appropriate virtual switch or not if you’re not ready to do this yet

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Use your vhdx disk that’s the remaining result of your merging efforts

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When you boot that virtual machine you’ll see that all the text files are there. It’s as if you’ve deleted the checkpoints in the GUI and retained “NOW” in the vhdx.

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Last but not least, you can use PowerShell or even DiskPart for this but I found that most people in this pickle value a GUI. Use what you feel most comfortable with.

Thanks for reading and hope this helps someone. Do remember “big boy” rules apply. This is not safe, easy or obvious in each and every situation so you are responsible for everything you do in your environment. If your in to deep, way over your head, etc. call in some expert help.

3 Ways To Deal With Lingering Hyper-V Checkpoints Formerly Known as Snapshots

Lingering or phantom Hyper-V checkpoints or snapshots

Once in a while the merging of checkpoints, previously known as snapshots, in Hyper-V goes south. An example of this is when checkpoints are not cleaned up and the most recent avhdx or multiple of these remains in use as active virtual disk/still even as you don’t see them anymore as existing in the Hyper-V Manager UI for example. When that happens you can try looking at the situation via PowerShell to see if that show the same situation. Whatever the cause, once in while I come across virtual machines that have one or more avhdx (or avdh) active that aren’t supposed to be there anymore. In that case you have to do some manual housekeeping.

Now please, do not that in Windows Server 2012(R2) Hyper-V replica is using checkpoints and since Windows Server 2012 R2 backups also rely on this. Just because you see a snapshot you didn’t create intentionally, don’t automatically think they’re all phantoms. They might exits temporarily for good reason Winking smile. We’re talking about dealing with real lingering checkpoints.

Housekeeping

Housekeeping comes in a couple of variants form simply dusting of to industrial cleaning. Beware of the fact that the latter should never be a considered a routine operation. It’s not a normal situation. It’s a last ditch resort and perhaps you want to call support to make sure that you didn’t miss anything else.

Basically you have tree options. In order of the easiest & safest to do first these are:

  1. Create a new checkpoint and delete it. Often that process will take care of merging the other (older) lingering avhd/avhdx files as well. This is the easiest way to deal with it and it’s as safe as it gets. Hyper-V cleans up for you, you just had to give it a kick start so to speak.
  2. Shut down the VM and create a new checkpoint. Export that newly created checkpoint. Yes you can do that. This will create a nicely exported virtual machine that only has the relevant vhd/vhdx files and no more checkpoints (avhd/avhdx). Do note that this vhd/vhdx is dynamically expanding one. If that is not to your liking you’ll need to convert it to fixed. But other than that you can dump the old VM (don’t delete everything yet) and replace it by importing the one you just exported. For added security you could first copy the files for save guarding before you attempt this. image
  3. Do manual mergers. This is a more risky process & prone to mistakes. So please do this only on a copy of the files. That way you’ll give Microsoft Support Services a fighting change if things don’t work out or you make a mistake. Also note that in this case you get one or more final VHDX files which you’ll use to create a new virtual machine with to boot from. It’s very hands on.

So that’s the preferred order of things to try/do in regards to safety. The 3rd option, is the last resort. Don’t do it before you’ve tried options 1 and 2. And as said above, if you do need to go for option 3, do it on copies.If you’re unsure on how to proceed with any of this, get an expert involved.

There’s actually another option which is very save but not native to Hyper-V. In the running virtual machine which current state you want to preserve do a V2V using Disk2vhd v2.01. Easy and sort of idiot proof if such a thing exists.

In a next blog post I’ll walk you through the procedure for the 3rd option. So if this is your last resort you can have practiced it before you have to use it in anger. Bit please, if needed, and do make sure it’s really needed as discussed above, try 1 first. If that doesn’t do it. Then try option 2. If that also fails try option 3. Do not that for option 2 and 3 you will have to create a new virtual machine with the resulting VHDX, having the required settings documented will help in this case.