Lock Management; Fencing - Red Hat CLUSTER SUITE - FOR RHEL 4 Overview

6 Chapter 1. Red Hat Cluster Suite Overview
Figure 1-3. GULM 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, one of the following Red Hat Cluster Suite components operates as
the lock manager: DLM (Distributed Lock Manager) or GULM (Grand Unified Lock Man-
ager). 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). 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). 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).

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 one of the following programs accord-
ing to the type of cluster manager and lock manager that is configured: