Installation guide

distributed across all nodes in the cluster (refer to Figure 1.2, “CMAN/DLM Overview”). GFS and
CLVM use locks from the lock manager. GFS uses locks from the lock manager to synchronize
access to file system metadata (on shared storage). CLVM uses locks from the lock manager to
synchronize updates to LVM volumes and volume groups (also on shared storage).
3.3. Fencing
Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O
from shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing
through the fence daemon, fenced.
When CMAN determines that a node has failed, it communicates to other cluster-infrastructure
components that the node has failed. fenced, when notified of the failure, fences the failed
node. Other cluster-infrastructure components determine what actions to take — that is, they
perform any recovery that needs to done. For example, DLM and GFS, when notified of a node
failure, suspend activity until they detect that fenced has completed fencing the failed node.
Upon confirmation that the failed node is fenced, DLM and GFS perform recovery. DLM
releases locks of the failed node; GFS recovers the journal of the failed node.
The fencing program determines from the cluster configuration file which fencing method to use.
Two key elements in the cluster configuration file define a fencing method: fencing agent and
fencing device. The fencing program makes a call to a fencing agent specified in the cluster
configuration file. The fencing agent, in turn, fences the node via a fencing device. When
fencing is complete, the fencing program notifies the cluster manager.
Red Hat Cluster Suite provides a variety of fencing methods:
• Power fencing — A fencing method that uses a power controller to power off an inoperable
node.
• Fibre Channel switch fencing — A fencing method that disables the Fibre Channel port that
connects storage to an inoperable node.
• GNBD fencing — A fencing method that disables an inoperable node's access to a GNBD
server.
• Other fencing — Several other fencing methods that disable I/O or power of an inoperable
node, including IBM Bladecenters, PAP, DRAC/MC, HP ILO, IPMI, IBM RSA II, and others.
Figure 1.3, “Power Fencing Example” shows an example of power fencing. In the example, the
fencing program in node A causes the power controller to power off node D. Figure 1.4, “Fibre
Channel Switch Fencing Example” shows an example of Fibre Channel switch fencing. In the
example, the fencing program in node A causes the Fibre Channel switch to disable the port for
node D, disconnecting node D from storage.
Chapter 1. Red Hat Cluster Suite Overview
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