Failover Clustering (III)

Cluster Quorum Configurations

In simple terms, the quorum for a cluster is the number of elements that must be online for that cluster to continue running. Server clusters require a quorum resource to function and this, like any other resource, is a resource which can only be owned by one server at a time, and for which servers can negotiate for ownership. In effect, each element can cast one “vote” to determine whether the cluster continues running. The voting elements are nodes or, in some cases, a disk witness or file share witness. The quorum resource is used to store the definitive copy of the cluster configuration so that regardless of any sequence of failures, the cluster configuration will always remain consistent. Each voting element (with the exception of a file share witness) contains a copy of the cluster configuration, and the Cluster service works to keep all copies synchronized at all times.

When network problems occur, they can interfere with communication between cluster nodes. A small set of nodes might be able to communicate together across a functioning part of a network but not be able to communicate with a different set of nodes in another part of the network. This can cause serious issues. In this "split" situation, at least one of the sets of nodes must stop running as a cluster.
Negotiating for the quorum resource allows Server clusters to avoid "split-brain" situations where the servers are active and think the other servers are down.
To prevent the issues that are caused by a split in the cluster, the cluster software requires that any set of nodes running as a cluster must use a voting algorithm to determine whether, at a given time, that set has quorum. 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 (that is, a quorum). If the number drops below the majority, the cluster stops running. 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.

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