The below table show Cluster Quorum Models which give SharePoint Administrator insight which options will be appropriated the option.
The cluster contains nodes and resources, to consider these resources in High Available situation only if the nodes are up and running. The cluster required more than half of the nodes to be up and running otherwise the cluster will go down. Quorum for the cluster maintain the number of nodes (also could be disk witness or file share witness) that must be online for the cluster to be run also to prevent scenarios when nodes can’t have communicated with each other which cause each node to try to own the resources at the same time. By default, every node in a failover cluster has a vote to determine whether the cluster continues running or not.
The value ‘0’ means the node doesn’t have a vote. The value ‘1’ means the node has a vote.
Each node in a WSFC cluster participates in periodic heartbeat communication to share the node’s health status with the other nodes.
Cluster Quorum has four models, only first three are recommended to use:
- Node Majority: only Nodes can vote
- Node and File Share Majority: Nodes and File Share witness can vote
- Node and Disk Majority: Nodes and Disk witness can vote
- No Majority: Disk Only: Only disk witness can vote and this model used in prior to Windows 2003 which it was only supported disk witness quorum.
To understand the usage for these models let us assume the following examples:
- If you have 4 nodes which it’s equal to 4 votes, then if 1 node fail then the 3 remaining nodes which is more than half of the cluster nodes will stay running.
- If you have 4 nodes which it’s equal to 4 votes, then if 2 nodes fail then the 2 remaining nodes will go down because it’s not more than half of cluster nodes.
- To increase the high availability for the cluster with 4 nodes then we can add File Share or disk witness which can have a vote, so in this case if we have 4 nodes + 1 File Share or Disk which it’s equal 5 votes then if 1 node fail then the 4 remaining nodes will stay running and if 2 fail then the 3 remaining nodes will stay running, so by adding the File Share or Disk witnesses we increase the availability with cheap means without a need to purchase a server (node).
- If you have 2 nodes and there is no File Share or disk, then if one node goes down then the cluster will go down.
It’s recommended to have odd number of votes which they equal to more than half by the quorum calculation (Minimum 2 Nodes + File Share or Disk witnesses).
Which model to select?
By default, Failover Cluster manager picks the best model based on cluster configuration and nodes. If there is odd number of Nodes, then the cluster will select (Node Majority) and if there is even number of Nodes and also has File Share then the cluster will select (Node and File Share Majority) and if it’s disk then it will be (Node and Disk Majority). If there is even number of Nodes and no disk or file share witness, then the mode will be (Node Majority) with warning messages.
If the Cluster use No Majority: Disk Only, then in this case the cluster has only 1 vote and if the disk goes down then the whole cluster will go down.