Red Hat ENTERPRISE LINUX 4 Manual

Also see for ENTERPRISE LINUX 4 - DEVELOPER TOOLS GUIDE: Installation guide Installation guide Tuning guide Installation guide Manual

Contents

Chapter 1. Red Hat Cluster Suite Overview6Figure 1.3. GULM Overview1.3.2. Lock ManagementLock management is a common cluster-infrastructure service that provides a mechanism for othercluster infrastructure components to synchronize their access to shared resources. In a Red Hatcluster, 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 betweenthe two lock managers is that DLM is a distributed lock manager and GULM is a client-server lockmanager. DLM runs in each cluster node; lock management is distributed across all nodes in thecluster (refer to Figure 1.2, “CMAN/DLM Overview”). DLM can be the lock manager only in a clusterconfigured with CMAN as its cluster manager. GULM runs in nodes designated as GULM servernodes; lock management is centralized in the nodes designated as GULM server nodes. GULMserver nodes manage locks through GULM clients in the cluster nodes (refer to Figure 1.3, “GULMOverview”). 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. GFSuses 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. FencingFencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O fromshared storage, thus ensuring data integrity.The cluster infrastructure performs fencing through one of the following programs according to thetype of cluster manager and lock manager that is configured:• Configured with CMAN/DLM — fenced, the fence daemon, performs fencing.• Configured with GULM servers — GULM performs fencing.When the cluster manager determines that a node has failed, it communicates to other cluster-infrastructure components that the node has failed. The fencing program (either fenced or GULM),when notified of the failure, fences the failed node. Other cluster-infrastructure components determine