Installation guide

Figure 1.1. Red Hat Cluster Suite Introduction

Note

Figure 1.1, “Red Hat Cluster Suite Introduction” includes GFS, CLVM, and

GNBD, which are components that are part of an optional package and not part

of Red Hat Cluster Suite.

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

3.1. Cluster Management

Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation

for cluster manager) performs cluster management in Red Hat Cluster Suite for Red Hat

Enterprise Linux 5. CMAN is a distributed cluster manager and runs in each cluster node;

cluster management is distributed across all nodes in the cluster (refer to Figure 1.2,

“CMAN/DLM Overview”).

CMAN keeps track of cluster quorum by monitoring the count of cluster nodes. If more than half

the nodes are active, the cluster has quorum. If half the nodes (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.

Quorum is determined by communication of messages among cluster nodes via Ethernet.

Optionally, quorum can be determined by a combination of communicating messages via

Ethernet and through a quorum disk. For quorum via Ethernet, quorum consists of 50 percent of

Chapter 1. Red Hat Cluster Suite Overview