Cluster Management - Red Hat CLUSTER SUITE FOR ENTERPRISE LINUX 4.7 Overview

• Fencing
• Cluster configuration management

3.1. Cluster Management

Cluster management manages cluster quorum and cluster membership. One of the following
Red Hat Cluster Suite components performs cluster management: CMAN (an abbreviation for
cluster manager) or GULM (Grand Unified Lock Manager). CMAN operates as the cluster
manager if a cluster is configured to use DLM (Distributed Lock Manager) as the lock manager.
GULM operates as the cluster manager if a cluster is configured to use GULM as the lock
manager. The major difference between the two cluster managers is that CMAN is a distributed
cluster manager and GULM is a client-server cluster manager. CMAN runs in each cluster node;
cluster management is distributed across all nodes in the cluster (refer to
"CMAN/DLM Overview"). GULM runs in nodes designated as GULM server nodes; cluster
management is centralized in the nodes designated as GULM server nodes (refer to
"GULM Overview"). GULM server nodes manage the cluster through GULM clients in the cluster
nodes. With GULM, cluster management operates in a limited number of nodes: either one,
three, or five nodes configured as GULM servers.
The cluster manager keeps track of cluster quorum by monitoring the count of cluster nodes that
run cluster manager. (In a CMAN cluster, all cluster nodes run cluster manager; in a GULM
cluster only the GULM servers run cluster manager.) If more than half the nodes that run cluster
manager are active, the cluster has quorum. If half the nodes that run cluster manager (or
fewer) are active, the cluster does not have quorum, and all cluster activity is stopped. Cluster
quorum prevents the occurrence of a "split-brain" condition — a condition where two instances
of the same cluster are running. A split-brain condition would allow each cluster instance to
access cluster resources without knowledge of the other cluster instance, resulting in corrupted
cluster integrity.
In a CMAN cluster, quorum is determined by communication of heartbeats among cluster nodes
via Ethernet. Optionally, quorum can be determined by a combination of communicating
heartbeats via Ethernet and through a quorum disk. For quorum via Ethernet, quorum consists
of 50 percent of the node votes plus 1. For quorum via quorum disk, quorum consists of
user-specified conditions.
Note
In a CMAN cluster, by default each node has one quorum vote for establishing
quorum. Optionally, you can configure each node to have more than one vote.
In a GULM cluster, the quorum consists of a majority of nodes designated as GULM servers
according to the number of GULM servers configured:
Cluster Management
Figure 1.2,
Figure 1.3,
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