Fencing is a very important concept in computer clusters for HA (High Availability). A cluster sometimes detects that one of the nodes is behaving strangely and needs to remove it. This is called fencing and is commonly done with a STONITH resource. Fencing may be defined as a method to bring an HA cluster to a known state.
Every resource in a cluster has a state attached. For example:
resource r1 is started on node1. In an HA cluster, such a
state implies that
resource r1 is stopped on all nodes except
node1, because an HA cluster must make sure that every resource
may be started on only one node. Every node must report every change that
happens to a resource. The cluster state is thus a collection of resource
states and node states.
When the state of a node or resource cannot be established with certainty, fencing comes in. Even when the cluster is not aware of what is happening on a given node, fencing can ensure that the node does not run any important resources.