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  Oracle Database Tips by Donald Burleson

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

RAC Components

GCS resources comprise the concurrency control mechanism on data blocks. They include enqueues involving the transaction locks, table locks, library cache locks, and the dictionary cache locks. Global Enqueue resources are normally held for a very short time and then quickly released. For example, the TX locks are acquired whenever a transaction starts. They are released immediately after the transaction commits or rolls back. Fig. 7.3 shows the main groups of resources involved in the synchronization process.

Figure 7.3 Resources for Coordination

Now, we will look deeper into the three main components, the global cache service, global enqueue service, and the global resource directory.

Global Cache Service

The main function of the global cache service (GCS) is to track the status and location of the data blocks. Status is understood as the resource role and the resource mode. The GCS is the main mechanism by which cache coherency among the 'multiple caches' is maintained.

GCS maintains the modes for blocks in the global role and is responsible for block transfers between the instances. The LMS processes handle the GCS messages and carry out the bulk of the GCS processing. GCS resource coordination is explained in detail in later sections.

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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