The non-volatile state of the cluster is stored in the cluster configuration database, which is accessible to all active nodes in the cluster. A set of one or more storage elements holds the master copy of this database. This set is referred to as the quorum storage, because a majority, or quorum, of these elements has to be available in order for one or more configured nodes to form and operate a cluster.
The storage elements are either a single storage device that can be accessed by any configured node, or they are storage devices that are accessible only by the configured nodes themselves. In order to form a cluster, one or more configured nodes needs to control a majority of the storage elements.
A slight variation of this is another type of quorum, called Witness, which does not require that the storage element contain the full copy of the cluster configuration database. Witness quorum only maintains cluster information on a node that is separate from the configured nodes in a cluster (hence the "Witness" moniker) about which nodes have the most up-to-date version of the configuration data. The information stored on the Witness allows configured nodes to determine whether their locally stored copy is up-to-date and therefore is able to go forward with forming a cluster.
The following is a list of specific quorum types and their operating characteristics:
Shared Disk: A disk that is physically accessible by all configured nodes in the cluster that contains the master copy of the configuration data. The node that has acquired ownership of this disk is able to form a cluster.
Majority of Nodes: A majority of the configured nodes are available to form a cluster, using their locally stored copies of the configuration data.
Hybrid: A combination of the two preceding types, except that ownership of the disk by itself does not constitute the ability to form a cluster. This style is used in clusters with an even number of nodes in order to allow the cluster to form when the node majority is off by one.
Local: A variation of Shared Disk where the storage element is a local disk on the node. This type can only be used when there is one configured node in the cluster.
Witness: As described in the preceding paragraph.
The nonvolatile cluster state contains the quorum configuration of the cluster.