A cluster quorum disk is the storage medium on which the configuration database is stored for a cluster computing network. The cluster configuration database, also called the quorum, tells the cluster which physical server(s) should be active at any given time. The quorum disk comprises a shared block device that allows concurrent read/write access by all nodes in a cluster.
In networking, clustering is the use of multiple servers (computers) to form what appears to users as a single highly available system. A Web page request is sent to a "manager" server, which then determines which of several other servers to forward the request for handling. Cluster computing is used to load-balance the traffic on high-traffic Web sites. Load balancing involves dividing the work up among multiple servers so that users get served faster.
Although clusters comprise multiple servers, users or other computers see any given cluster as a single virtual server. The physical servers themselves are called cluster nodes. The quorum tells the cluster which node should be active at any given time, and intervenes if communications fail between cluster nodes by determining which set of nodes gets to run the application at hand. The set of nodes with the quorum keeps running the application, while the other set of nodes is taken out of service.