About Quorum

About Quorum

In short quorum is minimum number of votes required for majority. the nodes participating in the windows cluster are connected through a private network and communicate through User Datagram Protocol (UDP) port 3343.The quorum configuration in a failover cluster determines the number of failures (failure of nodes) that the cluster can sustain while still remain online. If additional failure happened beyond this threshold, the cluster will stop running.Quorum is designed to handle the Split Brain scenario. 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.

Quorum role in cluster

When both nodes of a cluster are up and running, participating in their relevant roles (active and passive) they communicate with each other over the network. For example, if you change a configuration setting on the active node, this configuration change is automatically sent to the passive node and the same change made. This generally occurs very quickly, and ensures that both nodes are synchronized.

But, as you might imagine, it is possible that you could make a change on the active node, but before the change is sent over the network and the same change made on the passive node (which will become the active node after the failover), that the active node fails, and the change never gets to the passive node. Depending on the nature of the change, this could cause problems, even causing both nodes of the cluster to fail.

To prevent this from happening, a SQL Server 2005 cluster uses what is called a quorum, which is stored on the quorum drive of the shared array. A quorum is essentially a log file, similar in concept to database logs. Its purpose is to record any change made on the active node, and should any change recorded here not get to the passive node because the active node has failed and cannot send the change to the passive node over the network, then the passive node, when it becomes the active node, can read the quorum file and find out what the change was, and then make the change before it becomes the new active node.

In order for this to work, the quorum file must reside on what is called the quorum drive. A quorum drive is a logical drive on the shared array devoted to the function of storing the quorum.

Quorum models

Windows 2008 cluster supports four quorum models.

1 Node Majority 

2 Node and Disk Majority 

3 Node and File Share Majority

4 No Majority (disk only)

Node Majority: Node majority option is recommended for cluster with odd number of nodes.This configuration can handle a loss of half of the number of cluster nodes rounded off downwards. For example , a five node cluster can handle failure of two nodes. In this scenario three of the nodes (N1,N2,N3) can communicate each other but other two(N4 and N5) are not able to communicate. The group constituted by three node have the quorum (majority) and cluster will remain active and cluster service will be stopped on the other two nodes (N4 and N5). The resource group (sql server instance) hosted on that two nodes goes offline and come online on one of the three nodes based on possible owner settings.

Node and Disk Majority: This option is recommended for cluster with even number of nodes.In this configuration every node gets one vote and witness disk (quorum disk) gets one vote which makes total votes a odd number. The witness disk is a small ( approx 1 GB ) clustered disk.This disk is highly available and can failover between nodes. It is considered as part of the cluster core resource group.In a four node cluster, if there is a partition between two subsets of nodes, one of the subset will have witness disk and that subset will have quorum and cluster will remain online. This means that the cluster can lose any two voters,whether they are two nodes or one node and the witness disk.

Node and File Share Majority: This configuration is similar to the the Node and Disk Majority, but in this case the witness disk is replaced with a file share which is also known as File Share Witness Resource (FSW). This quorum configuration usually used in multi-site clusters (nodes are in different physical location) or where there is no common storage. The File Share Witness resource is a file share in any server in the same active directory which all the cluster nodes have access to. One of the node in the cluster will place a lock on the the file share to consider that node as owner of the file share.When this node goes offline or lost the connectivity another node grabs the lock and own the file share.On a standalone sever, the file share is not highly available , however the file share can also put on a clustered file share on an independent cluster,making the FSW clustered and giving it the ability to fail over between node. It is important that, this file share should not put in a node of the same cluster, because losing that node would cause for loosing two votes. A FSW does not store cluster configuration data like witness disk. It contain information about which version of the cluster configuration database is most recent.

No Majority (Disk only) : This configuration was available in windows server 2003 and has been maintained for compatibility reason and it is highly recommended not to use this configuration. In this configuration,only witness disk has a vote and there are no other voters in the cluster. That means if all nodes are online and able to communicate , but when witness disk failed or corrupted, the entire cluster will go offline.This is considered as single point of failure.