It’s not as simple as renaming the avhdx to vhdx

This arrives in via the feedback option on my blog

Hi. I see through your website that you are an expert in vhdx / avhdx file. I had a system crash with data loss. I think this data is in an avhdx file. When I rename this file in vhdx, I can mount it but I have an error: the file is corrupted. Do you know a procedure to repair this type of file? I thank you in advance for your support!

Oh dear! An expert? While flattery can get you a long way in life with certain people virtual disks are impervious to that sort of thing. Look, MVP, Veeam Vanguard, Dell Rockstar … tip of the spear, edge of the sword, it’s all fine and well but it’s no good to split a granite piece of rock and virtual disks don’t care about titles, jut about how they are designed to work.

Before we dive into some more details please use the comments sections under the relevant blog post to ask questions. That way everyone can benefit form the answer. It’s all quite anonymous if you want it to be. Secondly vendors like Microsoft have great public support forums with many thousand pairs of eyes reading. That might also work better and faster for your needs.

Some details

When you have avhdx your data is stored in the avhdx and in the parent disks (more avhdx but at least always one vhdx). While you can throw away what’s in a avhdx under certain conditions (and lose that data) and mount the vhdx you cannot throw away the vhdx and hope to be able to access the data in the avhdx you rename to vhdx.


For a case of real data corruption, not just phantom or mixed up VHDX/AVHDX chain, where you can try to intervene, even manually if needed – and if you have the skills – you’ll have to recover or restore data.

If the storage on which the vhdx/avhdx reside is corrupted a good but time-consuming run of chksdk /f /r can do the job. I have done that before with success. But there are no guarantees in this game.

Other than that, or when the storage is gone, it is restore time. This can be leveraging whatever backup solution you use or VSS snapshots on the storage side of things. Those options are your best bet. You can find some more info on manually manipulating vhdx/avhdx files here but that’s not what you’re facing here it seems.

If you don’t have recovery options in place, what can I say?

Stop what you’re doing and contact a good data recovery company. Only damage can come from trying if you don’t know what you’re doing. You can hope trial and error will fix it but that would be the triumph of hope over experience. You’re usually not that lucky. Trust me.

The snarky bit

I’ll fight like hell if I’m in a pickle and the data is valuable. But it’s near to impossible to do it for someone else as it’s hard, time consuming and often it’s a case were the files have been worked on before, so they tend to be messed up. If the data is not that valuable, just eat the loss.

In reality my time always seems less valuable then peoples their data . Now if you say you can help me retire early by trying anyway and are OK with a best effort, no guarantees given deal I might do it. But I’m pretty sure investing in backups and restores is way cheaper and will lead to better results. Your data is important and valuable, even when my time is not. Just saying

Latency kills


I was investigating a very problematic Windows Server 2016 Hyper-V cluster. That cluster was just performing horribly. “Everything” was hanging, stalling, crashing and RHS.exe errors where flying around while WER dumps got created by the dozen. Things were extremely slow up to the points functionality was just failing. The “fun” thing was that the cluster validation wizard while slow gave that cluster a big thumb up and a supported status as all was well.

Prying around

Time to pry around a bit and see if we could find something wrong. We save live migrations stall, fail, last forever in pending or get stuck at a certain percentage, sometimes finally succeeding with ridiculous blackout times. We could not open up virtual machine properties or very slowly. The FCI GUI was highly unresponsive but so was the Hyper-V Manager GUI or even PowerShell. Those were hanging at even loading the virtual machines or enumerating them with Get-VM. Everything was slow to the point it timed out or crashed. Restarting the services (Cluster, Hyper-V) didn’t do anything and restarting VMMS was super slow or just got stuck. It was a depressing sight for which people tended to blame Hyper-V / Microsoft.

As the title gives away it was latency. Not just ordinary high latency. Real bad latency. That kind of latency kills. Extreme latency produces symptoms that are similar to bugs or corrupt components of roles and features. We have a tendency to look at those first in the event logs and then we look at the network and its usual suspects (VMQ, SET, DCB). But nothing pointed to an issue that I could find.

So, storage maybe?Well we did find one Hyper-V host in the cluster with one HBA port producing too many error so we disabled that FC port for testing. No joy the Hyper-V cluster after a clean reboot of all nodes remained problematic. So on to the storage array itself.

Well holy smoke! On the two volumes for CSV in those cluster we saw latencies that were so bad I could not even believe a single VM would boot. It actually made my appreciation for Hyper-V and clustering grow as it managed to do at least a couple of things. With such latencies I would expect the services to just crash & call it a day.


The horrific latency on one of the CSV LUNs.

Looking at the logs we saw that the latencies occurred on the FC HBAs of the controllers. Each one above 50ms, peaking to 150-250ms and one huge peak at almost 500ms. We saw this on all four HBA’s.


The latency on one of the 4 FC HBA’s on one of the controllers. Not a good day. All HBAs had high latencies like this.

The issues were not at the host level (host HBA’s) or not even at the IOPS/bandwidth level of the storage itself. The latency for some reason was spiking. Further investigation lead to the conclusion that the issue was related to synchronous replication going totally wrong. Moving the replication mode to asynchronous fixed that. We’re now investigating why this happened and how to prevent this from happening again. But that’s another story.


Latency on one of the 4 FC HBAs on one of the controllers after we fixed the issue.

Do not assume anything

So, there you go. Everything depends on everything in some direct or indirect way. It’s all connected and that my friends, is why I’m a proponent of “service resilience engineering” where the responsible team owns the entire stack. That’s is how you can act fast.

My favorite deployment for VMs with Discrete Device Assignment for GPU


Recently I had an interesting discussion on how to leverage Discrete Device Assignment (DDA) for GPU needs when it’s only needed for a certain number of virtual machines. Someone had read my blogs on leveraging DDA that made here optimistic and enthusiastic. But she noticed in the lab she could not leverage DDA on a VM running on a cluster and she could not use storage QoS policies on a stand-alone Hyper-V host with local storage. So, what could she do?

Well for one, her findings are correct. Microsoft did not enable DDA on clustered virtual machines. It doesn’t make sense as the GPU hardware is tied to the virtual machine and any high availability, both planned (live migration) or unplanned (failover) isn’t possible and available anyway. It just cannot be done. I hear you, when you say “but they pulled it off for SR-IOV for networking”. Sure, but please keep in mind that network cards with Ethernet and TCP/IP allows for different approaches than high end video.

My favorite deployment for VMs with Discrete Device Assignment for GPU

My favorite deployment for VMs with Discrete Device Assignment (DDA) for GPU leveraged SMB3 SOFS shares for the virtual hard disks and stand-alone Hyper-V hosts that are member servers in the domain. Let me explain why.

Based on what we discussed above we have some options. One work around is running the DDA virtual machines not high available on local storage on a cluster node. But that would mean you would have a few VMs on all the nodes and that all those nodes must have a DDA capable GPU. Or if you limit the number of nodes that have such a GPU you’ll have a few odd balls in your cluster. You’ll need to manage some extra complexity and must save guard against assigning a GPU via DDA that is already in use for RemoteFX. That cause all kinds of unpleasantness, nothing too deadly but not something you want to do if on your production VDI clusters for fun. It’s a bit like not running a domain controller on a CSV and not making it highly available. If that’s the only option you have you can do that, and I do when needed as Microsoft has improved a lot of things to make this a better and less risky experience. But I prefer to have either physical one or host it on a separate non-clustered Hyper-V host if that’s an option because not all storage solutions and environments have all capabilities needed to make that fool proof.

Also note that running other storage on a S2D node isn’t supported. You have your OS on the boot disks and the disks used in storage spaces. Odd ones out aren’t supposed to be there as S2D will try to recruit them. You can get do it when using traditional shared storage

What I also don’t like about that is that if the cluster storage is not SMB3 SOFS you don’t get the benefit of storage QoS policies in Windows Server 2016, that only works with CSV. So optionally you could leave the non-clustered VM on a CSV. But that’s perhaps a bit confusing and some people might think some forgot to make the machine high available etc.

My preferred setup to get high available storage for virtual machines with DDA needs that benefits from what storage QoS polies have to offer for VDI is to use standalone Hyper-V hosts that have DDA capable GPUs and leverage SMB3 SOFS shares for the virtual Machines.


The virtual machines cannot be high available anyway so you lose nothing there. The beauty is that in this case, as you leverage a Windows Server 2016 SOFS cluster for Hyper-V storage over SMB3 shares, you do get Storage QoS policies.

#On a SOFS node

Get-StorageQosPolicy -Name DedicatedTier1Policy | Get-StorageQosFlow | ft InitiatorName, *IOPS, Status, PolicyID, filePath -AutoSize

#Query for the VM disks on the Hyper-V node

Get-VM -Name DDAVMSOFSStorage -ComputerName RemoteFXHost | Get-VMHardDiskDrive |fl *


#We generate some IO and get some stats on a SOFS node


get-storageQoSVolume -Mountpoint C:\ClusterStorage\SOFSDEMO\

get-storageQoSVolume -Mountpoint C:\ClusterStorage\SOFSDEMO\ | fl


You can start out with one Hyper-V node and add more when needed, that scale out. Depending on the needs of the virtual machines and specs of the servers (Memory, CPU cores) and the capability and number of GPU in the video cards you get some scale up as well.

To learn more about DDA go here:

To learn more about storage QoS policies go here:

Some more considerations

By going disaggregated. You can leverage a SOFS share for both virtual machines running on a Hyper-V cluster or on stand-alone (non-clustered) Hyper-V that are domain members. The SOFS cluster can be leveraging S2D, traditional storage spaces with shared SAS (JBODs) or even a FC, iSCSI or shared SAS SANS if that the only option you have. That’s all OK as long as it’s SOFS running on Windows Server 2016 and the Hyper-V hosts (stand alone or clustered) are a running 2016 as well (needed for Storage QoS policies and DDA). There is no need for the Hyper-V host to be part of a cluster to get the best results you need. If I use SOFS for both scenarios I can use the same storage array, but I don’t need to. I could also use separate storage arrays. If the Hyper-V cluster is leveraging CSV instead of SOFS I will need to use a separate one for SOFS as its ill advised to mix Hyper-V workloads with the SOFS role. Keep things easy, clear and supportable. I’ll borrow a picture I got from a Microsoft PM recently, do seek out the bad ideas.


Software-Defined Data Infrastructure Essentials

The last few months I spent some of my down time and commute time reading a book. A paper one actually. It’s Greg Schulz’s “Software-Defined Data Infrastructure Essentials”. It is as the sub title states about cloud, converged and virtual fundamental server storage I/O tradecraft.

It is not a book you’ll read to learn about a particular technology, product or vendor. It is a more holistic approach to educating people in todays IT landscape. That vast area of expertise in which all the considerations around storage in a modern IT environment come together. Where old and new, established and emerging ways of handling storage IO for a variety of  use cases meet and mix.


Reading the book helps to become more well versed in the subject and takes us out of our product or problem specific cocoons. That the main reason I’d recommend anyone to read it. I’m impressed by how well Greg managed to write a book on such a diverse subject that is accessible to all levels of expertise.The depth and the breadth of this subject make this quite a feat. On top of that this book is usable and valuable to both novice and experienced  professionals. I have said it before (on Twitter), but if I was teaching IT classes and needed to bring the student up to date in regards to the software defined cloud data center data considerations this would be the text book. It acknowledges the diversity of solutions and architectures in the real world and doesn’t make bold marketing statements. Instead it focuses on what you need to know and consider when discussing and designing solutions. I wish many IT manager, consultant and analyst would attend my fictional class but I’d settle for them reading this book and learning about a big part of what they need to manage, It would serve them well and help understand concerns other involved parties might want to see addressed.

For me an extra benefit was that I enjoy talking shop with Greg but I only get those opportunities on rare occasions during conferences.  As such, this book gave me some more time to read his views and insights. That’s the best next thing.