Red Hat CLUSTER SUITE FOR ENTERPRISE LINUX 4.7, Overview

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Figure 1.3. GULM 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, one of the following Red Hat Cluster Suite components operates as the lock
manager: DLM (Distributed Lock Manager) or GULM (Grand Unified Lock Manager). The major
difference between the two lock managers is that DLM is a distributed lock manager and GULM
is a client-server lock manager. DLM runs in each cluster node; lock management is distributed
across all nodes in the cluster (refer to Figure 1.2, “CMAN/DLM Overview” ). DLM can be the
lock manager only in a cluster configured with CMAN as its cluster manager. GULM runs in
nodes designated as GULM server nodes; lock management is centralized in the nodes
designated as GULM server nodes. GULM server nodes manage locks through GULM clients in
the cluster nodes (refer to Figure 1.3, “GULM Overview” ). With GULM, lock management
operates in a limited number of nodes: either one, three, or five nodes configured as GULM
servers. 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 one of the following programs according to
the type of cluster manager and lock manager that is configured:
Lock Management
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