Installation guide

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

Note

The maximum number of nodes supported in a Red Hat Cluster is 16.

1.3.1. Clust er 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 the node votes plus 1. For

quorum via quorum disk, quorum consists of user-specified conditions.

Note

By default, each node has one quorum vote. Optionally, you can configure each node to have

more than one vote.

CMAN keeps track of membership by monitoring messages from other cluster nodes. When cluster

membership changes, the cluster manager notifies the other infrastructure components, which then

take appropriate action. For example, if node A joins a cluster and mounts a GFS file system that

nodes B and C have already mounted, then an additional journal and lock management is required

for node A to use that GFS file system. If a cluster node does not transmit a message within a

prescribed amount of time, the cluster manager removes the node from the cluster and communicates

to other cluster infrastructure components that the node is not a member. Again, other cluster

infrastructure components determine what actions to take upon notification that node is no longer a

cluster member. For example, Fencing would fence the node that is no longer a member.

⁠Chapt er 1 . Red Hat Clust er Suit e Overview