Correcting the permissions on the folder with VHDS files & checkpoints for host level Hyper-V guest cluster backups


It’s not a secret that while guest clustering with VHDSets works very well. We’ve had some struggles in regards to host level backups however. Right now I leverage Veeam Agent for Windows (VAW) to do in guest backups. The most recent versions of VAW support Windows Failover Clustering. I’d love to leverage host level backups but I was struggling to make this reliable for quite a while. As it turned out recently there are some virtual machine permission issues involved we need to fix. Both Microsoft and Veeam have published guidance on this in a KB article. We automated correcting the permissions on the folder with VHDS files & checkpoints for host level Hyper-V guest cluster backup

The KB articles

Early August Microsoft published KB article with all the tips when thins fail Errors when backing up VMs that belong to a guest cluster in Windows. Veeam also recapitulated on the needed conditions and setting to leverage guest clustering and performing host level backups. The Veeam article is Backing up Hyper-V guest cluster based on VHD set. Read these articles carefully and make sure all you need to do has been done.

For some reason another prerequisite is not mentioned in these articles. It is however discussed in ConfigStoreRootPath cluster parameter is not defined and here You will need to set this to make proper Hyper-V collections needed for recovery checkpoints on VHD Sets. It is a very unknown setting with very little documentation.

But the big news here is fixing a permissions related issue!

The latest addition in the list of attention points is a permission issue. These permissions are not correct by default for the guest cluster VMs shared files. This leads to the hard to pin point error.

Error Event 19100 Hyper-V-VMMS 19100 ‘BackupVM’ background disk merge failed to complete: General access denied error (0x80070005). To fix this issue, the folder that holds the VHDS files and their snapshot files must be modified to give the VMMS process additional permissions. To do this, follow these steps for correcting the permissions on the folder with VHDS files & checkpoints for host level Hyper-V guest cluster backup.

Determine the GUIDS of all VMs that use the folder. To do this, start PowerShell as administrator, and then run the following command:

get-vm | fl name, id
Output example:
Name : BackupVM
Id : d3599536-222a-4d6e-bb10-a6019c3f2b9b

Name : BackupVM2
Id : a0af7903-94b4-4a2c-b3b3-16050d5f80f

For each VM GUID, assign the VMMS process full control by running the following command:
icacls <Folder with VHDS> /grant “NT VIRTUAL MACHINE\<VM GUID>”:(OI)F

icacls “c:\ClusterStorage\Volume1\SharedClusterDisk” /grant “NT VIRTUAL MACHINE\a0af7903-94b4-4a2c-b3b3-16050d5f80f2”:(OI)F
icacls “c:\ClusterStorage\Volume1\SharedClusterDisk” /grant “NT VIRTUAL MACHINE\d3599536-222a-4d6e-bb10-a6019c3f2b9b”:(OI)F

My little PowerShell script

As the above is tedious manual labor with a lot of copy pasting. This is time consuming and tedious at best. With larger guest clusters the probability of mistakes increases. To fix this we write a PowerShell script to handle this for us.

Below is an example of the output of this script. It provides some feedback on what is happening.

Correcting the permissions on the folder with VHDS files & checkpoints for host level Hyper-V guest cluster backup

Correcting the permissions on the folder with VHDS files & checkpoints for host level Hyper-V guest cluster backup

PowerShell for the win. This saves you some searching and typing and potentially making some mistakes along the way. Have fun. More testing is underway to make sure things are now predictable and stable. We’ll share our findings with you.

Frustrations about host level backups of Hyper-V guest clusters with Windows Server 2016


With Windows Server 2016 came the hope and promise of improved backups for Hyper-V environments. And indeed Microsoft delivered on that and has given us faster, more scalable and more reliable backups. With VHD sets also came the promise of host based backups for guest clusters.

The problem is that this promise or, as it is perhaps better to be mild and careful, that expectation has not been met. Decent, robust host based backups of guest clusters in Windows Server 2016 are still not a reality. For me this means it blocked a few scenarios and we’re working on alternatives. This is a missed opportunity I think for MSFT to excel at virtualization.

The problem

Doing host based backup of guest clusters with VHD Set disks is supported in Windows Server 2016 under certain conditions.

At RTM it became clear that CSV inside the guest cluster was not supported.

You need a healthy cluster with all disks one line

These requirements are reflected in Errors discovered during backup of VHDS in guest clusters

Error code: ‘32768’. Failed to create checkpoint on collection ‘Hyper-V Collection’

Reason: We failed to query the cluster service inside the Guest VM. Check that cluster feature is installed and running.

Error code: ‘32770’. Active-active access is not supported for the shared VHDX in VM group

Reason: The VHD Set disk is used as a Cluster Shared Volume. This cannot be checkpointed

Error code: ‘32775’. More than one VM claimed to be the owner of shared VHDX in VM group ‘Hyper-V Collection’

Reason: Actually we test if the VHDS is used by exactly one owner. So having 0 owner also creates this error. The reason was that the shared drive was offline in the guest cluster

Unfortunately, this is not the only problems people are facing. Quite often the backup software doesn’t support backing up VHD Sets or when it does they fail. Some of those failings like being unable to checkpoint the VHD Set have been addressed via Windows Updates. But there are others issues.

Let’s look at the two most common ones.

Issue 1

You can make one backup an all subsequent backups fail. This is due to the avhdx files being in used and locked. This means that as long as the cluster is up and running the recovery checkpoint chain keeps growing. This can be “cleaned” or merged but only by taking down the cluster.

At the first backup live seems good.


The recovery checkpoint as a collection is indeed working.


All attempts at another backup fail.


Shutting down all cluster VMs and starting them up again does merge the recovery checkpoints.

Issue 2

You can make backups, successfully but the recovery checkpoints never get merged. clip_image007

This sounds “better” but it isn’t. There is no way to merge the checkpoint. Manually merging the checkpoints of a VHD Set is bad voodoo.

Both situations get you into problems and I have found no solution so far. At the time or writing I’m back at the “never ending” recovery checkpoint chain situation. But that can change back to the 1st issue I guess. Sigh.

I have found no solution so far

For now I have been unable to solve these problem. There is no fix or even a workaround. The only to get out if this stale mate is to shut down every node of the guest clusters and then restart them all. Just a restart of the guest nodes of the cluster doesn’t do the trick of releasing the checkpoints files and merging them. While this allows you to take one backup successfully again, the problem returns immediately. For you reference that was my issue with the October 2017 CU (KB)

The other scenario we run into is that the backups do work but the recovery checkpoints never ever merge. Not even when you shut down the all the guest VM cluster nodes and start them. With frequent backup that turns into a disaster of a never ending chain of recovery checkpoints. This is actually the situation I was in again after the November 2017 updates on both guests & hosts (KB4049065: Update for Windows Server 2016 for x64-based Systems and KB4048953: 2017-11 Cumulative Update for Windows Server 2016 for x64-based Systems).

To me this situation is blocking the use of guest clustering with VHD Sets where a backup is required. For many reasons we do not wish to go the route of iSCSI or vFC to the guest. That doesn’t cut it for us.


Host level backups of guest clusters in Windows Server 2016 are still a no go. This despite the good hopes we had with VHD Sets to address this limitation and which we were eagerly awaiting. For many of us this is a show stopper for the successful virtualization guest clusters. Every month we try again and we’re not getting anywhere. Hence the frustration and the disappointment.

More than 1 year after Windows Server 2016 RTM we still cannot do consistent host level backup a Hyper-V guest cluster, not even those without CSV, but also not those with standard clustered disks. Trust me on the fact that many of us have given this feedback to Microsoft. They know and I suggest you keep voicing your concerns to them in order to keep it on their radar screen and higher on the priority list. You can do this by opening support calls and by asking for it on user voice. Please Microsoft, we need these workloads to be first class citizens. I’m clearly not the only unhappy camper out there as noticeable in various support forums: Cannot create checkpoint when shared vhdset (.vhds) is used by VM – ‘not part of a checkpoint collection’ error and Backing up a Windows Failover Cluster with Shared vhdx?

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.


You can then mount it on the host.


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.


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


Rename the virtual disk back to a avhdx extension.


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


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.

A first look at shared virtual disks in Windows Server 2016

Introduction to shared virtual disks in Windows Server 2016

Time to take a first look at shared virtual disks in Windows Server 2016 and how they are set up. Shared VHDX was first introduced in Windows Server 2012 R2. It provides shared storage for use by virtual machines without having to “break through” the virtualization layer. This way is still available to us in Windows Server 2016. The benefit of this is that you will not be forced to upgrade your Windows Server 2012 R2 guest clusters when you move them to Windows Server 2016 Hyper-V cluster hosts.

The new way is based on a VHD Set. This is a vhds virtual hard disk file of 260 MB and a fixed or dynamically expanding avhdx which contains the actual data. This is the “backing storage file” in Microsoft speak. The vhds file is used to handle the coordination of actions on the shared disk between the guest cluster nodes?

Note that an avhdx is often associated with a differencing disk or checkpoints. But the “a” stands for “automatic”. This means the virtual disk file can be manipulated by the hypervisor and you shouldn’t really do anything with it. As a matter of fact, you can rename this off line avhdx file to vhdx, mount it and get to the data. Whether this virtual disk is fixed or dynamically expanding doesn’t matter.

You can create on in the GUI where it’s just a new option in the New Virtual Hard Disk Wizard.

Or via PowerShell in the way you’re used to with the only difference being that you specify vhds as the virtual disk extension.

In both cases both vhds and avhdx are created for you, you do not need to specify this.

You just add it to all nodes of the guest cluster by selecting a “Shared Drive” to add to a SCSI controller …

… browsing to the vhds , selecting it and applying the settings to the virtual machine. Do this for all guest cluster nodes

Naturally PowerShell is your friend, simple and efficient.

Rules & Restrictions

As before shared virtual disk files have to be attached to a vSCSI controller in the virtual machines that access it and it needs to be stored on a CSV. Both block level storage or a SMB 3 file share on a Scale Out File Server will do for this purpose. If you don’t store the shared VHDX or VHD Set on a CSV you’ll get an error.

Sure for lab purposes you can use an non high available SMB 3 share “simulating” a real SOFS share but that’s only good for your lab or laptop.

The virtual machines will see this shared VHDX as shared storage and as such it can be used as cluster storage. This is an awesome concept as it does away with iSCSI or virtual FC to the virtual machines in an attempt to get shared storage when SMB 3 via SOFS is not an option for some reason. Shared VHDX introduces operational ease as it avoids the complexities and drawbacks of not using virtual disks with iSCSI or vFC.

In Windows Server 2012 R2 we did miss some capabilities and features we have come to love and leverage with virtual hard disks in Hyper-V. The reason for this was the complexity involved in coordinating such storage actions across all the virtual machines accessing it. These virtual machines might be running on different hosts and, potentially the shared VHDX could reside on different CSVs. The big four limitations that proved to be show stopper for some use cases are in my personal order of importance:

  1. No host level backup
  2. No on line dynamic resize
  3. No storage live migration
  4. No checkpoints
  5. No Hyper-V Replica support

I’m happy to report most of these limitations have been taken care of in Windows Server 2016. We can do host level backups. We can online resize a shared VHDX and we have support for Hyper-V replica.

Currently in 2016 TPv4 storage live migration and checkpoints (both production and standard checkpoints) are still missing in action but who knows what Microsoft is working on or has planned. To the best of my knowledge they have a pretty good understanding of what’s needed, what should have priority and what needs to be planned in. We’ll see.

Other good news is that shared VHDX works with the new storage resiliency feature in Windows Server 2016. See Virtual Machine Storage Resiliency in Windows Server 2016 for more information. Due to the nature of clustering when a virtual machine loses access to a shared VHDX the workload (role) will move to another guest cluster node that still has access to the shared VHDX. Naturally if the cause of the storage outage is host cluster wide (the storage fabric or storage array is toast) this will not help, but other than that it provides for a good experience. The virtual machine guest cluster node that has lost storage doesn’t go into critical pause but keeps polling to see if it regains access to the shared VHDX. When it does it’s reattached and that VM becomes a happy fully functional node again.

It also supports the new Storage Qos Policies in Windows Server 2016, which is something I’ve found during testing.

Thanks for reading!