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  Oracle Database Tips by Donald Burleson

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

Cache Coherency

GCS synchronizes global cache access, allowing only one instance at a time to modify the block. Thus, cache coherency is maintained in the RAC system by coordinating buffer caches located on separate instances. GCS ensures that the data blocks cached in different cache buffers are maintained globally. That is why some people prefer to call cache fusion a 'diskless cache coherency' mechanism. This is true in a sense, because the previous Oracle parallel server version (OPS) utilized 'forced disk writes' to maintain cache coherency.

Global Cache Service

GCS is the main controlling process for cache fusion. It tracks the location and status (mode and role) of the data blocks, as well as the access privileges of the various instances. GCS guarantees data integrity by employing global access levels. It maintains block modes for data blocks in the global role. It is also responsible for block transfers between instances. As shown in Fig. 7.2, upon a request from an instance, GCS organizes the block shipping and the appropriate lock mode conversions. Various background processes, such as global cache service processes (LMSn) and the global enqueues service daemon (LMD), implement the global cache service.


Figure 7.2 Message/Resource Exchange controlled by GCS

Before going further into a detailed discussion of the cache fusion mechanism and how GCS operations are performed in different scenarios, let's look at and recap some basic SGA structures and locking concepts in the next section.

We have covered more details on the SGA structures in Chapter 4

SGA Components and Locking

The Oracle database is accessed through an instance. The combination of SGA (System Global Area) with one or more Oracle processes constitutes an instance. After the instance is started, the database is associated with it. This process is called 'database mounting'. In the case of a RAC system, the database can be associated with multiple instances. The main purpose of the SGA is to store data in memory for quick access and for processing.

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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