Installation guide

4 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 follow-
ing Red Hat Cluster Suite components performs cluster management: CMAN (an abbre-
viation 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 Figure 1-2). GULM runs in nodes designated as GULM server nodes;
cluster management is centralized in the nodes designated as GULM server nodes (refer to
Figure 1-3). 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 communi-
cating heartbeats via Ethernet and through a quorum disk. For quorum via Ethernet, quo-
rum consists of 50 percent of the node votes plus 1. For quorum via quorum disk, quorum
consists of user-specified conditions.