Cluster Infrastructure; Cluster Management - Red Hat ENTERPRISE LINUX 4 Manual

Chapter 1. Red Hat Cluster Suite Overview

1.3. Cluster Infrastructure

The Red Hat Cluster Suite cluster infrastructure provides the basic functions for a group of computers
(called nodes or members) to work together as a cluster. Once a cluster is formed using the cluster
infrastructure, you can use other Red Hat Cluster Suite components to suit your clustering needs (for
example, setting up a cluster for sharing files on a GFS file system or setting up service failover). The
cluster infrastructure performs the following functions:

• Cluster management

• Lock management
• Fencing
• Cluster configuration management
1.3.1. 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
designated as GULM server nodes; cluster management is centralized in the nodes designated as
GULM server nodes (refer to
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:
4
Figure 1.2, "CMAN/DLM
Figure 1.3, "GULM Overview"). GULM server nodes manage the cluster
Overview"). GULM runs in nodes