Lock Management; Fencing - Red Hat Enterprise Linux 5 Cluster Suite Overview

Lock Management

Figure 1.2. CMAN/DLM Overview
1.3.2. Lock Management
Lock management is a common cluster-infrastructure service that provides a mechanism for other
cluster infrastructure components to synchronize their access to shared resources. In a Red Hat
cluster, DLM (Distributed Lock Manager) is the lock manager. As implied in its name, DLM is a
distributed lock manager and runs in each cluster node; lock management is distributed across all
Figure 1.2, "CMAN/DLM Overview"). GFS and CLVM use locks from the
nodes in the cluster (refer to
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).

1.3.3. 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:
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