What is Quorum? – SQL Server Clustering

When creating a cluster in Windows, the Failover Cluster Manager will indicate which quorum configuration best suits your configuration. The importance of this choice is often overlooked.

Before we go into the possible configurations, it’s helpful to understand the function of a quorum first.

What is a quorum?

A cluster quorum is a configuration database for a cluster computing network. Quorum tells the cluster which physical server(s) should be active at any given time.

“Quorum” is incredibly important to keep your SQL Server online when you use Windows Failover Clustering or AlwaysOn Availability Groups. Quorum is a voting mechanism to ensure correct ownership of shared resources.

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 Quorum? - SQL Server ClusteringWhy quorum is necessary?

Quorum is designed to handle the Split Brain scenario. Cluster nodes communicate with each other over the network (port 3343). When nodes are unable to communicate each other, each node assume that, resource groups owned by other nodes have to brought online. When same resource brought online on multiple nodes at the same time, data corruption can occur. This scenario is called Split Brain. To prevent Split Brains we need to bring the cluster resource online on a single node (rather than multiple nodes).

Let us assume that we have four node cluster and one instance of sql server is running on each node. Node1 and Node2 lost the communication with Node3 and Node4. Node1 and Node2 can communicate each other and  Node3 and Node4 can communicate each other. In this scenario each group does not know what happened to other  two nodes. Are they offline or just a communication failure ?. In this scenario, Node1 and Node2 try to bring online the Sql instance(resource) owned by Node3 and Node4. In the same way Node3 and Node4 will try to bring online the Sql instance (resource) owned by the Node1 and Node2, which will lead to disk corruption and many other issues. The windows cluster quorum setting is designed to prevent this kind of scenario. By having the concept of quorum, the cluster will force the cluster service to stop in one of the subsets of nodes to ensure that there is only one true owner for the particular resource group.

Quorum configuration choices

You can choose from among four possible quorum configurations:

What is Quorum? - SQL Server Clustering1. Node Majority (recommended for clusters with an odd number of nodes)
Can sustain failures of half the nodes (rounding up) minus one. For example, a seven node cluster can sustain three node failures.

2. Node and Disk Majority (recommended for clusters with an even number of nodes)
Can sustain failures of half the nodes (rounding up) if the disk witness remains online. For example, a six node cluster in which the disk witness is online could sustain three node failures.

Can sustain failures of half the nodes (rounding up) minus one if the disk witness goes offline or fails. For example, a six node cluster with a failed disk witness could sustain two (3-1=2) node failures.

3. Node and File Share Majority (for clusters with special configurations)
Works in a similar way to Node and Disk Majority, but instead of a disk witness, this cluster uses a file share witness.

Note that if you use Node and File Share Majority, at least one of the available cluster nodes must contain a current copy of the cluster configuration before you can start the cluster. Otherwise, you must force the starting of the cluster through a particular node. For more information, see “Additional considerations” in Start or Stop the Cluster Service on a Cluster Node.

4. No Majority: Disk Only (not recommended)
Can sustain failures of all nodes except one (if the disk is online). However, this configuration is not recommended because the disk might be a single point of failure.

Click here to see What is a Failover Cluster?

Click here to see High Availability Options in SQL Server 2008

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