Cluster Management - Red Hat CLUSTER SUITE FOR ENTERPRISE LINUX 5.1 Overview

• 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
"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.
Cluster Management
Figure 1.2,
5