The topic of quorum in Microsoft Failover Clustering
High-availability cluster
High-availability clusters (also known as HA clusters or fail-over clusters) are groups of computers that support server applications that can be reliably utilized with a minimum of down-time. They operate by using high availability software to harness redundant computers in groups or clusters that provide continued service when system components fail.
What is quorum in clustering?
Quorum is an important concept to understand in clustering. For a cluster to be active, it needs some method of keeping track of which node is the current owner of the cluster object. How this is achieved depends on the number of nodes that will be in your cluster.
What is Windows server failover clustering quorum and Hyper-V?
Windows Failover Clustering is able to protect production workloads running on top of multiple hosts by utilizing a special means for establishing node majority in the Windows Failover Cluster, called quorum. In this post, we will take a look at Windows Server Failover Clustering Quorum and how it relates to the Hyper-V.
What is the purpose of quorum in Hyper V?
The Purpose and Function of Quorum. Quorum has one basic purpose: to ensure that protected roles can always find their way to one, and only one, live host. If you’ve ever wondered why Hyper-V Replica doesn’t have any built-in automatic failover capability, this is why.
What is quorum in Hyper V?
Quorum guides the actions of surviving nodes when there are system-level failures. This technology has evolved substantially over the last few versions of Windows/Hyper-V Server and many are not aware of the changes.
What is quorum used for?
Quorum is designed to prevent split-brain scenarios which can happen when there is a partition in the network and subsets of nodes cannot communicate with each other. This can cause both subsets of nodes to try to own the workload and write to the same disk which can lead to numerous problems.
What is a quorum model?
A quorum is the minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation in a distributed system. A quorum-based technique is implemented to enforce consistent operation in a distributed system.
What is the purpose of quorum Drive in clustering?
The quorum configuration in a failover cluster determines the number of failures that the cluster can sustain while still remaining online. If an additional failure occurs beyond this threshold, the cluster will stop running.
What is a quorum for 13 members?
A quorum is the minimum number of members that must be present for a group to conduct business. For the Charter Review Committee of thirteen (13) members, this means that a minimum of seven (7) members be present for the Committee to take action.
How is a quorum calculated?
To determine a quorum, start by determining the number of people in your organization who have decision-making powers for the organization. Then, calculate how many people in that group you'd need to agree to have a majority, which is equal to the quorum.
What is quorum node?
A node quorum is the minimum number of nodes that must be running in order for the daemon to start. Node quorum is the default quorum algorithm for GPFS™. For more information on node quorum, see the section on Quorum , in the IBM Spectrum Scale: Concepts, Planning, and Installation Guide.
What is quorum server?
The cluster configuration database, also called the quorum, tells the cluster which physical server(s) should be active at any given time. The quorum disk comprises a shared block device that allows concurrent read/write access by all nodes in a cluster.
What will happen if quorum disk fails?
If the quorum disk fails, the cluster service on ALL nodes in the cluster will stop since none of the nodes will be able to successfully arbitrate for the quorum resource.
What happens when cluster loses quorum?
If the cluster loses a node and it loses quorum, it goes offline again because it is no longer in the forced state. To bring it back online when it does not have quorum requires forcing the cluster to start without quorum.
What is quorum majority?
The Open Meeting Law defines a quorum as a simple majority of the members of a public body, unless otherwise provided in a general or special law, executive order, or other authorizing provision.
What is quorum in failover clustering?
Quorum guides the actions of surviving nodes when there are system-level failures. This technology has evolved substantially over the last few versions of Windows/Hyper-V Server and many are not aware of the changes.
Why does Quorum have only one host?
Quorum has one basic purpose: to ensure that protected roles can always find their way to one, and only one, live host. If you’ve ever wondered why Hyper-V Replica doesn’t have any built-in automatic failover capability, this is why. It’s simple enough for the Replica host to know that it can’t reach the source host anymore, but that’s not enough to be certain that it isn’t still up. If it were to simply start the replicas, it’s entirely possible that the source and the replica machines would all be running simultaneously. This is known as a split-brain situation. In some cases that might not be a big problem, but in others it could be catastrophic. One example would be a database system in which some clients can see the source virtual machine while others have switched over to using the replica. Updates between these two systems would be difficult, if not impossible, to reconcile.
What to do if a cluster fails?
If too many nodes fail and the entire cluster goes down, you’ll have a few more steps to take. First, you need enough nodes to maintain quorum. You can evict permanently failed nodes or fix them. Once fixed, you can use Failover Cluster Manager to start the cluster or the Start-Cluster PowerShell cmdlet.
How many votes are there in a node majority?
In node majority, each node gets a vote. So, for a four node cluster in node majority plus disk witness, there are a total of five votes. As long as any given node can contact two other nodes or one node and the disk witness, it will remain online.
What is quorum agreement?
Oftentimes, quorum is explained and understood as a continual agreement made between the majority of nodes to exclude any that are not responding, sort of the way humans determine quorum at meetings. While there is some truth to this in the more recent versions, it does not tell the entire tale.
Why manually manage votes?
The purpose of manually managing votes is so that you can guide how loss of quorum will affect your hosts. This is most commonly done when you have a geographically dispersed cluster as shown in the image above. You are essentially choosing which site will always stay online in the event that the inter-site link fails. This is often done most effectively by judicious placement of a witness. However, it’s also helpful as a plan against multiple failures. For instance, if Site A above were to lose one of its hosts prior to the link going down, Dynamic Quorum would be choosing who to take a vote away from. By removing one or more votes from Site B in advance, you are maintaining control.
Is Site B partitioned?
In the Microsoft documentation, Site B is considered a “partitioned” site (and if you look in the event logs for the cluster, you’ll find entries indicating that the networks are partitioned). You can manually get Site B back online while the link is down.
What is quorum configuration?
This quorum configuration is most commonly used since it works well with 2-node and 4-node clusters which are the most common deployments. This configuration is used when there is an even number of nodes in the cluster. In this configuration, every node gets 1 vote, and additionally 1 disk gets 1 vote, so there is generally an odd number of total votes.
How many votes does a cluster need to achieve quorum?
It is important to realize that the cluster requires more than half of the total votes to achieve quorum. This is to avoid having a ‘tie’ in the number of votes in a partition, since majority will always mean that the other partition has less than half the votes.
How does a quorum work?
Because a given cluster has a specific set of nodes and a specific quorum configuration, the cluster will know how many "votes" constitutes a majority of votes , or quorum. If the number of voters drop below the majority, the cluster service will stop on the nodes in that group. These nodes will still listen for the presence of other nodes, in case another node appears again on the network, but the nodes will not begin to function as a cluster until the quorum exists again.
What is disk only quorum?
The Disk Only quorum type was available in Windows Server 2003 and has been maintained for compatibility reasons, however it is strongly recommended to never use this mode unless directed by a storage vender. In this mode, only the Disk Witness contains a vote and there are no other voters in the cluster.
Why does a cluster stop running?
If an additional failure occurs beyond this threshold, the cluster will stop running. A common perception is that the reason why the cluster will stop running if too many failures occur is to prevent the remaining nodes from taking on too many workloads and having the hosts be overcommitted.
Do you need an odd number of nodes in a cluster?
Because of this logic, it is recommended to always have an odd number of total voters in the cluster. This does not necessarily mean an odd number of nodes is needed since both a disk or a file share can contribute a vote, depending on the quorum model. A voter can be:
How is the quorum of a cluster determined?
The quorum for a cluster is determined by the number of voting elements that must be part of active cluster membership for that cluster to start properly or continue running. For a more detailed explanation, see the understanding cluster and pool quorum doc.
What happens when you configure a quorum?
As a general rule when you configure a quorum, the voting elements in the cluster should be an odd number. Therefore, if the cluster contains an even number of voting nodes, you should configure a disk witness or a file share witness. The cluster will be able to sustain one additional node down.
What does it mean when a node is started with a setting that prevents quorum?
A node started with a setting that prevents quorum indicates to the Cluster service to join an existing running cluster instead of forming a new cluster instance. This prevents the remaining nodes from forming a split cluster that contains two competing instances.
How to force cluster to start on a specific node?
To force the cluster to start on a specific node that contains a cluster configuration that you want to use, you must use the Windows PowerShell cmdlets or equivalent command-line tools as presente d after this procedure.
What is a quorum witness?
The cluster quorum is the majority of voting nodes in the active cluster membership plus a witness vote. A quorum witness can be a designated disk witness or a designated file share witness.
Can you recover a cluster?
After you determine that you cannot recover your cluster by bringing the nodes or quorum witness to a healthy state, forcing your cluster to start becomes necessary. Forcing the cluster to start overrides your cluster quorum configuration settings and starts the cluster in ForceQuorum mode.
Can you enable dynamic quorum management in Server 2012?
In Windows Server 2012, as an advanced quorum configuration option, you can choose to enable dynamic quorum management by cluster. For more details on how dynamic quorum works, see this explanation.
How does cluster quorum work?
How cluster quorum works. When nodes fail, or when some subset of nodes loses contact with another subset, surviving nodes need to verify that they constitute the majority of the cluster to remain online. If they can't verify that, they'll go offline.
What is dynamic quorum?
Dynamic quorum enables the ability to assign a vote to a node dynamically to avoid losing the majority of votes and to allow the cluster to run with one node (known as last-man standing). Let's take a four-node cluster as an example. Assume that quorum requires 3 votes.
What is dynamic quorum behavior?
Dynamic quorum behavior. If you have an even number of nodes and no witness, one node gets its vote zeroed. For example, only three of the four nodes get votes, so the total number of votes is three, and two survivors with votes are considered a majority.
What is failover clustering?
Windows Server Failover Clustering provides high availability for workloads running on Azure Stack HCI and Windows Server clusters . These resources are considered highly available if the nodes that host resources are up; however, the cluster generally requires more than half the nodes to be running, which is known as having quorum.
Why is Quorum used?
Quorum is designed to prevent split-brain scenarios which can happen when there is a partition in the network and subsets of nodes cannot communicate with each other. This can cause both subsets of nodes to try to own the workload and write to the same disk which can lead to numerous problems.
Does a cluster stay up if you lose 3 nodes?
So, with dynamic quorum, the cluster will stay up even if you lose three nodes. The above scenario applies to a general cluster that doesn't have Storage Spaces Direct enabled. However, when Storage Spaces Direct is enabled, the cluster can only support two node failures.
Does pool have dynamic quorum?
The pool resource owner (active cluster node) can be the +1. But pool quorum works differently from cluster quorum in the following ways: the pool uses one node in the cluster as a witness as a tie-breaker to survive half of drives gone (this node that is the pool resource owner) the pool does NOT have dynamic quorum.
What is dynamic quorum?
Dynamic quorum is the new ability of the Windows Failover Cluster service to adjust the vote of remaining active nodes to ensure that quorum can be maintained in the event of yet another node failure or shutdown. Starting with Windows Server 2012, dynamic quorum is enabled by default. This is helpful in situations where the quorum requirements of a cluster are changing frequently. Dynamic quorum allows Failover Clustering to change the vote in case there is a tied vote where one vote is stripped from a node in question. Microsoft recommends to always use a disk or file share witness when deploying Windows Server 2012 R2 clusters and higher.
What is failover cluster?
Windows Server Failover Cluster technology backing Microsoft Hyper-V provides a powerful mechanism to withstand failures, provide high-availability, distribute resources, and also protect against “split-brain” scenarios. Split-brain is a very real problem that can lead to very odd cluster behavior and possibly corruption of data. With the quorum functionality found in Windows Server Failover Cluster services, Microsoft has provided a way for cluster nodes to determine if a node is able to participate in the Failover Cluster. The node majority mechanism is a simple way for odd numbered host clusters to simply look at the majority of reachable nodes and determine if quorum is met. With new versions of Windows Server, Microsoft has introduced new functionality such as dynamic quorum and the cloud witness to further extend the ability of Windows Server Failover Cluster quorum functionality to meet the demands of today’s multi-datacenter, stretched clusters and maintain the availability of the Failover Cluster resources.
What is the mechanism for Windows Server failover cluster?
To resolve the Windows Server Failover Cluster split-brain scenario, Microsoft has introduced a mechanism called Quorum which allows Windows Server Failover Clusters to resolve the potential issue of an “even vote” among Cluster nodes.
Can one host access a virtual machine?
The resources in question are virtual machines. Fortunately, in most situations, only one host will be able to gain access to a virtual machine disk. However, split-brain can lead to all kinds of failover and failback scenarios that can lead to resources flapping up and down.
Question
Hello, I want to implement a Hyper-V Cluster with 2 Microsoft Hyper-V Server 2008 R2 and a HP SAN. I know that I have to create a quorum disk for the cluster but what's about the size? How much MB or GB should I use for the Quorum disk? Regards!
Answers
Use a small Logical Unit Number (LUN) that is at least 512 MB in size. <--- So here is the Microsoft Recommendation
The Purpose and Function of Quorum
How to Think of Quorum
- Oftentimes, quorum is explained and understood as a continual agreement made between the majority of nodes to exclude any that are not responding, sort of the way humans determine quorum at meetings. While there is some truth to this in the more recent versions, it does not tell the entire tale. It is more correct to think of quorum always being calculated from the perspectiv…
Through 2008 R2
- My personal experience starts with 2008 R2. I know that in very early versions, only disk witnessoptions were available. By the time of 2008 R2, a number of new options were available. The complete list is: 1. Disk Witness Only: This is the original version of quorum. One cluster disk is selected to serve as the witness. If any node cannot contact the witness, it assumes quorum …
Changes in 2012
- Prior to 2012, quorum was static. Every item in the cluster is considered to have a vote. If the quorum mode is witness-only, then there is only one vote, and if it is lost, then the entire cluster and all nodes in it will stop. In node majority, each node gets a vote. So, for a four node cluster in node majority plus disk witness, there are a total of five votes. As long as any given node can co…
Changes in 2012 R2
- 2012 R2 brought about almost as many improvements as the original introduction of dynamic quorum. The witness diskis now assigned a dynamic vote as well. In 2012, only the nodes were dynamic. If a witness was present, it always had a vote. The big change with this is that it’s always recommended to have a witness regardless of the number of nodes in the cluster. The cluster c…
Recovering from Failure
- Sometimes, nodes fail. That’s one of the reasons you have a cluster in the first place. Quorum will do its job and shut down nodes that cannot maintain quorum. For the most generic sense, this will work as you expect. You’ve got three nodes in your one and only site and node 2 dies. Nodes 1 and 3 will pick up the pieces and proceed along. In 2008 R2, node 2 will still have its vote, so if n…
Configuring Quorum
- There are PowerShell cmdlets available for basic quorum configuration in 2012+, but I’m just going to link to them (2012 here, 2012 R2 here). It’s better and easier to use Failover Cluster Manager. Right-click the cluster’s name at the top of the tree in the left pane, hover over More Actions, and click Configure Cluster Quorum settings to get started. In 2012 and beyond, you’ll g…
Understanding Quorum
- The quorum for a cluster is determined by the number of voting elements that must be part of active cluster membership for that cluster to start properly or continue running. For a more detailed explanation, see the understanding cluster and pool quorum doc.
Quorum Configuration Options
- The quorum model in Windows Server is flexible. If you need to modify the quorum configuration for your cluster, you can use the Configure Cluster Quorum Wizard or the FailoverClusters Windows PowerShell cmdlets. For steps and considerations to configure the quorum, see Configure the cluster quorumlater in this topic. The following table lists the three quorum config…
General Recommendations For Quorum Configuration
- The cluster software automatically configures the quorum for a new cluster, based on the number of nodes configured and the availability of shared storage. This is usually the most appropriate quorum configuration for that cluster. However, it is a good idea to review the quorum configuration after the cluster is created, before placing the cluster into production. To view the …
Configure The Cluster Quorum
- You can configure the cluster quorum settings by using Failover Cluster Manager or the FailoverClusters Windows PowerShell cmdlets.
Recover A Cluster by Starting Without Quorum
- A cluster that does not have enough quorum votes will not start. As a first step, you should always confirm the cluster quorum configuration and investigate why the cluster no longer has quorum. This might happen if you have nodes that stopped responding, or if the primary site is not reachable in a multisite cluster. After you identify the root cause for the cluster failure, you can u…
Quorum Considerations For Disaster Recovery Configurations
- This section summarizes characteristics and quorum configurations for two multisite cluster configurations in disaster recovery deployments. The quorum configuration guidelines differ depending on if you need automatic failover or manual failover for workloads between the sites. Your configuration usually depends on the service level agreements (SLAs) that are in place in y…
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