A Windows File Share Witness is a file share that is available to all nodes in a high availability (HA) cluster. The job of the Witness is to provide an additional quorum vote when necessary in order to ensure that a cluster continues to run in the event of a site outage.
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A File Share Witness is most commonly used when a high availability cluster spans two or more data center sites and when there is an even number of total nodes. In high availability clustering, a common strategy to ensure surviving nodes continue to run or takeover the work of failed nodes is to employ a quorum system. The quorum system gives every node a "vote." Cluster operations can only continue if a majority of nodes are available, thus ensuring the remaining nodes aren't overloaded. If a quorum cannot be met, cluster operations will cease.
Consider the example of a four node cluster with two nodes at each site. If one site experiences an outage, the remaining two nodes may be capable of carrying on, but would not have a quorum (having two of four possible votes). The theory behind a File Share Witness is to employ a third party – usually located in a third physical location – to serve as an additional vote so that a quorum can be reached and cluster operations can continue to run. Using this example, if one site were to fail, the File Share Witness located in a third location could confirm the operation of the remaining two nodes and cast a third vote to reach a quorum.
Another example might include a scenario in which network connectivity between cluster locations is disrupted. In this situation, all cluster nodes are capable of functioning, but should not continue to work independently. In this case, both sites would have independent connections to the File Share Witness. If connectivity between the two data center sites fails, the nodes at one site would query the File Share Witness to attempt to reach a quorum. If a quorum can be reached, those nodes can continue cluster operations. In this case, whichever group of nodes first queries the File Share System and reaches a quorum will continue running, while the other nodes will stop cluster operations.
A File Share Witness is similar to a cluster quorum disk, but does not store cluster configuration data. However, the file share does include information about which version of the cluster configuration database is most current.