Sizing a Majority Node Set Cluster (Server Clusters: Majority Node Set Quorum)

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP2

As described above, if a partition of a cluster loses quorum, all of the resources on that partition of the cluster are terminated. This is done to guarantee the cluster semantics that only a single instance of the resource will be brought online (and thus serving or modifying data). Loss of quorum not only happens when a cluster is partitioned, but in the event of node failures, a cluster can lose quorum simply because there are insufficient nodes up and running. The number of nodes in a majority node set cluster should be carefully considered when setting up a cluster. For example, in a two-node cluster, if either node fails for any reason, the other node cannot continue because it does not have quorum. In an eight-node cluster, up to three nodes can fail before the cluster loses quorum. The following table shows the number of node failures that a given quorums disk set cluster can tolerate and continue:

Number of nodes configured in the cluster	Number of node failures
1	0
2	0
3	1
4	1
5	2
6	2
7	3
8	3

In a situation where a cluster spans multiple sites, it is recommended that at least one site has a majority over the other. That way in case there is a site link failure you can be assured that the cluster is up and running on at least one site. If you do lose the majority site, the nodes in the minority can be restarted manually to form a cluster.