Quorum resource

In every cluster, a single resource is designated as the quorum resource. This resource maintains the configuration data necessary for recovery of the cluster. This data, in the form of recovery logs, contains details of all of the changes that have been applied to the cluster database. This provides node-independent storage for cluster configuration and state data.

Quorum resource and the cluster database

The cluster database is an integral part of the formation of a server cluster. When a node joins or forms a cluster, the Cluster service must update the node's private copy of the cluster database. When a node joins an existing cluster, the Cluster service can retrieve the data from the other active nodes. However, when a node forms a cluster, no other node is available. The Cluster service uses the quorum resource's recovery logs to update the node's cluster database. To ensure cluster unity, XOX and XOX use the quorum resource to ensure that only one set of active, communicating nodes is allowed to operate as a cluster. A node can form a cluster only if it can gain control of the quorum resource. A node can join a cluster or remain in an existing cluster only if it can communicate with the node that controls the quorum resource.

For more information on the cluster database, see Cluster database.

For more information on cluster resources, see Server cluster resources.

Scenarios

To illustrate the importance of the quorum resource, consider these situations in a simple two-node cluster:

• Node 1 fails. Node 2 continues operating, writing changes to the cluster database. But before node 1 can be restarted, node 2 fails. When node 1 becomes active, it must update its private copy of the cluster database with the changes made by node 2. The Cluster service uses the quorum resource's recovery logs to perform the update.
• The networks providing communication between nodes 1 and 2 fail. Each node assumes the other node has failed and attempts to continue operating as the cluster. If both could succeed, the result would be two separate clusters using the same cluster name and competing for the same resources. XOX and XOX use quorum resource ownership to prevent this problem and maintain cluster unity. In this scenario, the node that gains control of the quorum resource is allowed to form a cluster, and the other fails over its resources and becomes inactive.

The quorum resource can be any resource with the following attributes:

• The resource must enable a single node to gain physical control of it and defend its control.
• The resource must provide physical storage that can be accessed by any node in the cluster.
• The resource must use the NTFS file system.

Types of quorum resources

There are two resources that come with the operating system that can act as a quorum resource for multinode clusters. They are:

• The Physical Disk resource
• The Majority Node Set resource

For single-node clusters, the Local Quorum resource acts as a quorum resource. However, other developers can create their own quorum resource types for any resources that meet the arbitration and storage requirements. For more information, see the Microsoft Software Development Kit (SDK).