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Chapter 1. Red Hat Cluster Suite Overview
Figure 1.2. CMAN/DLM Overview
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 nodes in the cluster (refer to
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,
When CMAN determines that a node has failed, it communicates to other cluster-infrastructure
components that the node has 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
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
6
.
fenced
, when notified of the failure, fences the failed
fenced
fenced
Figure 1.2, "CMAN/DLM
has completed fencing the failed node.
Overview"). GFS and
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