A Red Hat cluster suite when configured for high availability

A Red Hat cluster suite when configured for high availability attempts to ensure

service availability by monitoring other nodes of the cluster. All nodes of the cluster

must agree on their configuration and shared services state before the cluster is

considered Quorate and services are able to be started.

The primary form of communicating node status is via a network device (commonly

Ethernet), although in the case of possible network failure, quorum can be decided

through secondary methods such as shared storage or multicast.

Software services, filesystems and network status can be monitored and controlled by

the cluster suite, services and resources can be failed over to other network nodes in

case of failure.

The Cluster suite forcibly terminates a cluster node's access to services or resources

to ensure the node and data is in a known state. The node is terminated by removing

power or access to the shared storage.

Service locking and control is guaranteed through fencing and STONITH, more recent

versions of Red Hat use a distributed lock manager (DLM), to allow fine grained

locking and no single point of failure. Earlier versions of the cluster suite relied on

GULM (Grand Unified Lock Manager) which could be clustered, but still presented a

point of failure) if the nodes acting as gulm servers were to fail. GULM as a locking

manager is available but deprecated in Red Hat Cluster Suite 5.