Red Hat ENTERPRISE LINUX 4 Manual page 15

Cluster Management
• Configured with one GULM server — Quorum equals one GULM server.
• Configured with three GULM servers — Quorum equals two GULM servers.
• Configured with five GULM servers — Quorum equals three GULM servers.
The cluster manager keeps track of membership by monitoring heartbeat 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 heartbeat 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.
Figure 1.2. CMAN/DLM Overview
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