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Donald K. Burleson

Oracle RAC Tips


Cluster software is usually equipped with monitoring functionality, failure detection, and cluster component management, including the servers and server connections. Cluster software specializes in failure detection through the use of software probes, fault isolation, and fault containment wherever applicable.

Cluster software is usually called Cluster Manager or Operating system dependent (OSD) layer, as in the case of Oracle Real Application Clusters. The cluster hardware manufacturer generally provides cluster software. However, in the case of the OSD layer for Oracle RAC system in Windows 2000 and Linux, Oracle provides the cluster software.

The failure detection process involves the determination of a fault condition and the application of a proper response. A failure could result in a switchover to a secondary component and may result in the total shutdown of the failing component, in which case some kind of reconfiguration takes place. As an example, if two interconnects are configured, the fault or loss of one would not cripple the cluster but merely result in temporarily shutting it off. If one of the nodes hangs, the cluster manager evicts the failed node and informs other members so that the application (database instance) can take the necessary reconfiguration action.

The cluster manager is also responsible for node monitoring, which entails polling the status of various resources like interconnect hardware/software, shared disks, oracle instances, and public networks. In the case of Oracle RAC, the cluster manager interacts with the global cache service to reconfigure the failed instance resources. The cluster manager interacts with RAC modules and helps to reconfigure the system resources as required.

Cluster software protects against the conditions of split-brain and amnesia. The split-brain condition occurs when all communication between nodes is lost and the cluster becomes partitioned into sub-clusters, each believing that it is the only partition. Amnesia occurs when the cluster restarts after a shutdown using cluster configuration data prior to shutdown. Changes in cluster membership drive the cluster reconfiguration sequence that may, in turn, result in services being migrated from failed or faulty nodes to healthy ones. More details about split-brain condition are covered in later sections.

For Linux clusters, Oracle provides cluster management software (OCMS). OCMS is part of Oracle Enterprise Edition for Linux. It manages cluster membership, maintains a global view of clusters, monitors cluster nodes, and performs cluster reconfiguration. OCMS is installed automatically when RAC is selected, and consists of components like hang check-timer, node monitor, and cluster manager.

In case of failover, cluster vendors usually supply agents or bundled scripts to control and manage the recovery process. Agents are available for most of the popular databases such as Oracle, Sybase, and DB2. Agents follow preconfigured procedures to restart the database on the surviving node.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.  


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