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  Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Locking Mechanism

In the case of a RAC system, new and improved components or services namely, the Global Cache Service (GCS) and Global Enqueue Service (GES)) handle the lock and access management functions better than the earlier Oracle Parallel Server distributed lock manager (DLM). GES is discussed in more detail later in this chapter.

RAC Components

When Oracle reads a data block into the cache, it opens a GCS resource to coordinate concurrent access by multiple instances. Oracle coordinates and converts the resource into different modes and roles, depending on the following:

The data block accessed will be modified or read.

A data block exists in the cache of only one instance or in multiple caches.

Thus, a 'resource' is a concurrency control mechanism on the data blocks. It is also called a 'GCS resource'

In a stand-alone Oracle instance, various locks are used to control data integrity and concurrency. Similarly, the multi-instance RAC architecture deals with cached data block mode roles and the controlling access levels. The cached data blocks acquire a global nature.

The above text is an excerpt from:

Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
ISBN 0-9744355-4-6

by Mike Ault, Madhu Tumma


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