Tag Archives: Quorum

Cluster Shared Volumes without Active Directory

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Introduction

Cluster Shared Volumes without Active Directory will come online if certain conditions are met. Basically, when the cluster can form. That’s what we’ll talk about here. There are a lot of things to consider when virtualizing your Active Directory environment, partially or completely. Too much for this blog post alone and much of it has been discussed before. Here I’ll discuss when people count on their Cluster Shared Volumes coming online when active directory is not available but are disappointed and think that it’s a bug or a broken promise. It is not! We know that since Windows Server 2012 the CSVs coming online do not depend on Active Directory being available (thanks to the local CLIUSR account being used for cluster starts).

Cluster Shared Volumes without Active Directory

So, let’s address getting your Cluster Shared Volumes online when AD is not available. The things to realize is that not having Active Directory has been taken care off. The thing that still can go wrong is that the cluster doesn’t come on line properly and that means that the CSV LUNs won’t be online as those are cluster resource. Hyper-V can boot the VMs without the cluster being formed but as the VMs reside on a CSV and those are not available, that’s what’s causing the problem. That means that getting your cluster to come up is the real issue.

Getting your cluster to come up is the real issue.

If cluster shared volumes can come up without a domain controller / AD being available since Windows Server 2012 (Failover Clustering and Active Directory Integration) how is it that some many people still have issues with it? How do you make this work for you? You can use a disk witness to protect you in most cases. When you have a file share or cloud witness you can take some step to avoid running into this issue.

There is a great article on cluster behavior here: Failover Cluster Node Startup Order in Windows Server 2012 R2 (the same rules apply for Windows Server 2016). Read it and you’ll notice that the behavior differs depending on whether you have a witness defined and whether that witness is a witness disk or a file share or cloud witness. The disk witness has a copy of the cluster database and if available will help you out of any situation where the paxos on the disk witness (if it’s available) is more recent then the one on the cluster node as the node will download the cluster database info from the disk witness. But a file share or cloud witness don’t have a copy, so they have a small disadvantage under certain scenarios. CSV’s that won’t come up is when booting the nodes in the wrong order when using a file share or cloud witness is one of those scenarios but not having AD isn’t the root cause of this.

Some people will never notice this issue at all, especially not when they have disk witness, but when they do, it might be at a very inconvenient moment in time. Examples of where this situation can occur are a single cluster environment where the domain controllers are running on CSV and high available in that cluster that was shut down completely, the cluster has a file share witness and the nodes are started in the wrong order.

Making sure your CSVs come online = clustering being formed

Well for one make sure you are using Windows Server 2012 or higher for your cluster. That’s a given.

Beyond that you basically just need to know what to do in what scenario to get your cluster up and running so you won’t have issues with getting the CSV to come up. You just have to follow some rules of thumb and you’ll be fine. Also, there’s almost always a way to get out of pickle, just don’t panic. But also remember that you can make your live easier when you design your solutions with failure in mind and by knowing your options so you can act correctly. I my example I’ll be using Hyper-V cluster with CSVs but the same goes for SOFS, SQL Server clusters leveraging CSVs etc.

Planned down time

If you’re shutting down a complete cluster you have two options to make sure things go a smooth as possible.

Option 1 – A clean cluster shutdown by the book

  • Shut down the workload, i.e. the VMs.
  • Stop the cluster
  • Shutdown the Cluster nodes.
  • Boot the cluster nodes. You can start with any you like. The CSVs will come on line. You will be able to start the VMs. Do start with the domain controllers and wait for them to come on line before starting the others.

During this you’ll see some “collateral” events, errors, warnings due to Active Directory not being available. The cluster name has issues without Active Directory but that a management connection point, it doesn’t mean the cluster doesn’t work. Once Active Directory is available the cluster name will come on line automatically when the default failure policy restarts it. You can also manage the cluster via RDP or console by connection to “.” locally on that node or use the running node name. You can also try to bring cluster name on line manually when Active Directory is up and running.

Option 2 – A clean cluster shutdown as it happens most of the times in real life

Which is one a lot of people do to keep part of the work load running as long as possible.

  • Shut down the no critical workload, i.e. the VMs.
  • Pause a node so the critical workloads live migrate to other nodes
  • When the node is paused, shut down the node.
  • You rinse and repeat this until the last node is left with only the most critical VMs

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  • Finally, you stop the workload on that last cluster node and shut it down as well.
  • Now comes the critical part: Remember what node you shut down last. You have to start that one first and you’ll see that your CSV will come on line. If you boot another node, you might panic as the CSV will not come on line.

Now I need to correct this a little bit. With a disk witness you are OK whatever node you boot. When the paxos numbers on the first node to boot and the disk witness are compared the most recent copy will be used. Either the local one on the node will be used directly or after it has been updated with the data form the disk witness. To make things simple for the ops team I always tell them to note the last node they shut down anyway no matter what type of witness they have. It’s good info to have.

With a file share or cloud witness the shutdown/startup order really comes into play. The reason this happens is that by shutting down node by node we end up with a one node cluster (last man standing).  When that’s shut down that’s the only node that knows about the last (potential) changes in the cluster as it holds a copy of the cluster database. Remember that a files hare or a cloud witness has no copy of the cluster database. That’s why the last node to be shut down has to come on line first when comparing the paxos numbers with the witness as that node can form a cluster. If the node that boots first does not hold the most recent paxos number it cannot download the cluster databases info from the file share or cloud witness. Such a node cannot form a cluster and bring the CSVs online. If the first node to boot was the last one to shut down, it can form a cluster and the CSVs will come on line. This is the big difference with option one where you shut down the entire cluster and then take the nodes of line.

You might not know or remember the order. If that’s the case you still have options like starting the cluster node with the -fixquorum option (net.exe start clussvc /forcequorum) at the risk of loosing some cluster changes that are not in the local cluster database.

No need to go to immediately backups, extract the domain controller VMs from SAN snapshots or mount a LUN to a different machine to extract the VM files for the DC or so. Don’t panic!

One or more failed nodes

Well as long as the cluster survives your domain controller VMs should fail over. Keep ‘m on separate node (anti-affinity), separate CSV LUNs, if possible separate clusters if all domain controller virtual machines are going to be running high available on a cluster node and that cluster is still functional after all. No issues here.

Total cluster failure

The cluster nodes all show due to a “global” BSOD or are turned off due to a power failure or a storage array crash. This is more the realm of bad dreams I know, it does happen. Often things will recover and you’ll be fine but you can do your part. The same rules apply, get the cluster to form and you’ll have your CSVs on line. In a bad case -fixquorum is your friend but normally it’s not your first option. In the worst case you’ll need recovery from backup of the cluster or rebuild it. It’s a very bad day if it comes to that. And cluster recovery is not the subject of this post.

Conclusion

Don’t blame Active Directory and start troubleshooting or fixing the wrong problem. So yes, CSV will come on line when certain conditions are met and you can work yourself out of a pickle if needed. But during a disaster that’s only extra work and stress you might not want to worry about if you can avoid it. It’s good to know how to resolve issues around CSVs not coming online when the shit hits the fan as even the best laid out plans tend to get side tracked by reality when disaster strikes.

If you cannot guarantee control over all the prerequisites and might not have the skills in please when needed, you might consider other options. Some of these are actually the best practices of the past when a CSV would indeed not come on line without active directory in any way. This is great for AD related issues but not for you offline CSVs, they need the cluster to form properly!

Sure, you can run the domain controller virtual machines on local storage, and not made high available. This cloud be on one of the cluster nodes (*) or on a stand-alone Hyper-V host. Having a physical domain controller is also a possibility. This helps avoid issues with AD in virtualized environments as many other services are very dependent on them and it’s good to have on one available all of the time and get them back on line a.s.a.p..

I’ll leave you with the fact that virtualizing domain controllers can be done but it pays to study up on how to do it well and test your assumptions in the lab. There is a lot of information on virtualizing domain controllers for a reason. Read it and process what you’ve learned from it. You might find that this CCV thingies is not the most complex subject to deal with.

(*) Please note that some cluster deployments like HCI based on S2D do not support running other (local) storage in addition to the boot OS and the S2D storage pool volumes.

A First look at Cloud Witness

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Introduction

In Windows Server 2012 R2 Failover Clustering we have 2 types of witness:

  1. Disk witness: a shared disk that can be seen by all cluster nodes
  2. File Share Witness (FSW): An SMB 3 file share that is accessible by all cluster nodes

Since Windows Server 2012 R2 the recommendation is to always configure a witness. The reason for this is that thanks to dynamic quorum and dynamic witness. These two capabilities offer the best possible resiliency without administrator intervention and are enabled by default. The cluster dynamically assigns a quorum vote to node when it’s up and removes it when it’s down. Likewise, the witness is given a vote when it’s better to have a witness, if you’re better off without the witness it won’t get a vote. That’s why Microsoft now advises to always set a witness, it will be managed automatically. The result of this is that you’ll get the best possible uptime for a cluster under any given circumstance.

This is still the case in Windows Server 2016 but Failover clustering does introduce a new option witness option: cloud witness.

Why do we need a cloud witness?

For certain scenarios such a cluster without shared storage and especially when a stretched cluster is involved you’ll have to use a FSW. It’s a great solution that works as well as a disk witness in most cases. Why do I say most? Well there is a scenario where a disk witness will provide better resiliency, but let’s not go there now.

Now the caveat here is that you’ll need to place the FSW in a 3rd independent site. That’s a hard order for many to fulfill. You can put in on the desktop of the receptionist at a branch office or on a virtual machine on the cluster itself but it’s “suboptimal”. Ideally the FSW is independent and high available not dependent on what it’s supposed to support in achieving quorum.

One of the other workarounds was to extend AD to Azure, deploy a SOFS Cluster with an non CA file share on a cluster of VMs in Azure and have both other sites have access to it over VPN or express route. That works but in a time of easy, fast, cheap and good solutions it’s still serious effort, just for a file share.

As Microsoft has more and more use cases that require a FSW (site aware stretched clusters, Storage Spaces Direct, Exchange DAG, SQL Availability Groups, workgroup or multi domain clusters) they had to find a solution for the growing number of customers that do not have a 3rd site but do need a FSW. The cloud idea above is great but the implementation isn’t the best as it’s rather complex and expensive. Bar using virtual machines you can’t use Azure file services in the cloud as those are primarily for consumption by applications and the security is done via not via ACLing but access keys. That means the security for the Cluster Name Object (CNO) can’t’ be set. So even when you can expose a cloud file to on premises to Windows 2016 (any OS that supports SMB 3 actually) by mapping it via NET USE the cluster GUI can’t set the required security for the cluster nodes so it will fail. And no you can’t set it manually either. Just to prove this I tried it for you to save you the trouble. Do NOT even go there!

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So what is possible? Well come Windows Server 2016 Failover Clustering now has a 3rd type of witness. The cloud witness. Functionally wise it’s like a FSW. The big difference it’s a dedicated, cloud based solution that mitigates the need and costs for a 3rd data center and avoids the cost of the workarounds people came up with.

Implementing the cloud witness

In your Azure subscription you create a storage account, for this purpose I’ve create one named democloudwitness in my resource group RG-Demo. I’m using a separate storage account to keep thing tidy and separated from my other demo storage accounts.

A storage account gets two Access keys and two connection strings. The reason for this is that we you need to regenerate the keys you can have your workloads use the other one this can be done without down time.

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In Azure the work is actually already done. The rest will happen on premises on the cluster. We’ll configure the cluster with a witness. In PowerShell this is a one liner.

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If you get an error, make sure the information is a correct and you can reach Azure of HTTPS over the internet, VPN or Express Route. You normally do not to use the endpoint parameter, just in the rare case you need to specify a different Azure service endpoint.

The above access key is a fake one by the way, just so you know. Once you’re done Get-ClusterQuorum returns Cloud Witness as QuorumResource.

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In the GUI you’ll see

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When you open up the Blobs services in your storage account you’ll see that a blob service has been created with a name of msft-cloud-witness. When you select it you’ll see a file with a GUID as the name.

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That guid is actually the same as your cluster instance ID that you can find in the registry of your cluster nodes under the HKLM\Cluster key in the string value ClusterInstanceID.

Your storage account can be used for multiple clusters. You’ll just see extra entries each with their own guid.

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All this consumes so few resources it’s quite possibly the cheapest ever way of getting a cluster witness. Time will tell.

Things to consider

• Cloud Witness uses the HTTPS REST (NOT SMB 3) interface of the Azure Storage Account service. This means it requires the HTTPS port to be open on all cluster nodes to allow access over the internet. Alternatively an Azure Site-2-Site VPN or Express Route can be used. You’ll need one of those.

• REST means no ACLing for the CNO like on a SMB 3 FSW to be done. Security is handled automatically by the Failover Cluster which doesn’t store the actual access key, but generates a shared access security (SAS) token using the access key and stores it securely.

• The generated SAS Token is valid as long as the access key remains valid. When rotating the primary access key, it is important to first update the cloud witness (on all your clusters that are using that storage account) with the secondary access key before regenerating the primary access key.

• Plan your governance between cluster & Azure admins if these are not the same. I see Azure resources governance being neglected and as a cluster admin it’s nice to have some degree of control or say in the Azure part of the equation.

For completeness I’ll mention that the entire setup of a cloud witness is also very nicely integrated in to the Failover Cluster GUI.

Right click on the desired cluster and select “Configure Cluster Quorum Settings” from menu under “More Actions”

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Click through the startup form (unless you’ve never ever done this, then you might want to read it).

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Select either “Select the quorum witness” or “Advanced quorum configuration”

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We keep the default selection of all nodes.

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We select to “Configure a cloud witness”

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Type in your Azure storage account name, your primary access key for the “Azure storage account key” and leave the endpoint at its default. You’ll normally won’t need this unless you need to use a different Azure Service Endpoint.

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Click “Next”to review what you’re about to do

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Click Next again and let the wizard run.

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You’ll get a report when it’s done. If you get an error, make sure the information is a correct and you can reach Azure of HTTPS over the internet, VPN or Express Route.

Conclusion

I was pleasantly surprised by how it easy it was to set up a cloud witness. The biggest hurdle for some might be access to Azure in secured environments. The file itself contains no sensitive information at all and while a VPN or Express Route are secured connectivity options this might not be allowed or viable in certain environments. Other than this I have found it to be very reliable, effective cheap and easy. I really encourage you to test it and see what it can do for you.