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.


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.


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).

‘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.’.


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.


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).


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.


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)


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.






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”


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.


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.

The Mysterious Case of Infrequent Network Connectivity Issues on 2 Hyper-V VMs Out of 40 Guests

In The Mysterious Case of Infrequent Network Connectivity Issues on 2 Hyper-V VMs Out of 40 Guests I share a trouble shooting experience with you. I was asked if I could possibly take a look at a weird, but very infrequent network issue with 2 VMs (W2K12R2) on a cluster (W2K12R2) running over 40 guests? Sure! These 2 virtual machines worked well 98% of the time. About 2% of the time they just fell of the network, sometimes both vNICs, sometimes both VMs. Asking what they meant, they said unreachable. But we can’t find anything wrong as all other VMs run fine with the same configuration on the same hosts. They told me there was nothing in the event logs of either the host or the guests to explain any of this. A reboot or 2 or even a live migration sometimes fix the issue. Normally the monthly patch cycle prevent to many problems with connectivity. Pretty weird! Usually bad firmware, drivers or bad offload feature support can cause issues, but that would not target just 2 out of 40 VMs that have the same settings.

It was only these 2 VMs, not matter what host the were running on in the cluster. As the the vNICs shared the same 2 vSwitches (teamed) with all other VMS that never had issues I was pretty sure the configuration of the switches, NIC, teams and vSwitch were OK. This was verified for due diligence and it  checked out on all hosts as expected. All firmware, drivers and offloads were done correctly.

I also checking the VLANs settings of the vNIC themselves for those two VMs and compared them a couple of VMs that had no issues what so ever and found them to be identical.

At first everything seemed fine and I was stumped. The event logs both in the VMs as on the hosts were squeaky clean. After that exercise I started running some PowerShell command lets to take a look at the configuration of the VMs on the hosts. You see the GUI does not expose all possible configurations and I wanted to look every configuration option. That’s when I found the following


The vNIC for the 2 offending VMs were in Access mode while the VlanList had a single value 0 (basically meaning untagged, it’s a reserved VLAN for priority tagging and the use is not 100% standard across switch vendors). This just didn’t compute. In the GUI we did not see this, there things looked normal.


You cannot even set this in the GUI, it won’t allow you.



But when run in a PowerShell command it allows you to make this configuration. So maybe that’s what’s happened.

Set-VMNetworkAdaptervlan -VMName DNS01 -Access -VlanId 0

No one knew, nor can I tell you. But I tested to verify this does run and makes that configuration without any issue, weird. Anyway, I resolved the issue by running the following command.

Set-VMNetworkAdaptervlan -VMName DNS01 –Untagged


The rare connectivity issue disappeared and all was well in 100% of the cases. That how The Mysterious Case of Infrequent Network Connectivity Issues on 2 Hyper-V VMs Out of 40 Guests came to a happy end.

Hyper-V Virtual Machines and the Storage Optimizer

Windows Server 2012 (R2) has made many improvements to how storage optimization and maintenance is done. You can read a lot more about this in What’s New in Defrag for Windows Server 2012/2012R2. It boils down to a more intelligent approach depending on the capability of the underlying storage.

This is reflected in the Media type we see when we look at Optimize Drives.

This is my workstation … looks pretty correct a couple of SSDs and a couple of HDDs.


SSD are optimized intelligently by the way.When VSS is leveraged SSD do get fragmentation and so one in while they are “defragmented”. This has to do with keeping performance up to par. Read more about this in The real and complete story – Does Windows defragment your SSD? by Scott Hanselman.

The next example is a Hyper-V Cluster. You can see the local disks identified as HDD and the CSV as Thin provisioned disks. Makes sense to me, the SAN I use supports thin provisioned disks.


But now, let’s look at a Virtual Machine with virtual disks of every type known and on any type of storage we could find. All virtual disks are identified as “Thin provisioned disk”. How can that be?


What had me puzzled a little bit is that in a virtual machine each and every virtual disk is identified as thin provisioned disk. It doesn’t matter what type of virtual disk it is: fixed VHD/VHDX or dynamically expanding VHD/VHDX. It also doesn’t matter on what physical disk the virtual disk resides: SATA, SAS, SSD, SAN (iSCSI/FC) LUN or CSV, SMB Share …

So how does this work with a fixed VHD on a local SATA disk? A VHD doesn’t know about UNMAP, does it? And a SATA HHD? How does that compute? Well, my understanding on this is that all virtual disks, dynamically expanding or fixed, both VHDX/VHD are identified as thin provisioned disks, no matter what type of physical disk they reside on (CSV, SAS, SATA, SSD, shared/non shared). This is to allow for UNMAP (RETRIMs in Storage Optimizer speak, which is  way of dealing with the TRIM limitations / imperfections, again see Scott Hanselman’s blog for this) command to be sent from the guest to the Hyper-V storage stack below. If it’s a VHD those UNMAP command are basically black holed just like they would never be passed down to a local SATA HHD (on the host) that has no idea what it is and used for.

But wait a minute ….what about SSD and defragmentation you say, my VHDX lives on an SSD.. Well they are for one not identified as SSD or HDD. The hypervisors deals with the storage optimization at the virtual layer. The host OS handles the physical layer as intelligent as it can to optimize the disks as best as it can. How that happens depends on the actual storage beneath in the case of a modern SAN you’ll notice it’s also identified as a Thin provisioned disk. SANs or hyper converged storage arrays provide you with storage that is also virtual with all kinds of features and are often based on tier storage which will be a mix of SSD/SAS/NL-SAS and in some cases even NVMe Flash. So what would an OS have to identify it as?  The storage array must play its part in this.

So, if you ever wondered why that is, now you know. Hope you found this interesting!

E2EVC 2015 Berlin SMB Direct Slide Deck

I attended and presented at E2EVC 2015 in Berlin from June 12th to June 14th. The networking was a blast. No “marchitecure” bull shit or vendor fairy tales what so ever and lots of very open discussions on the realities we’re seeing and facing in virtualization and cloud. Most account managers and esoteric presales would die a painful (but fast) death in this environment.


One session was with my Hyper-V Amigo buddy Carsten Rachfahl and was pure demo extravaganza, so no slides. My own session was “SMB Direct – The Secret Decoder Ring” and was an attempt to position this technology what by looking at the why and where followed by the how by who and when.


I hope a lot of people had at least a better understanding of SMB Direct, RDMA and DCB. The second aim was to take away the fear many people have of this tech by showcasing it in short demos. Time constraints where a challenge so it was not a 200 level session.

Please download the presentation here if interested.

Enjoy. If you have any concerns or questions, ask, and I’ll try to answer.