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

 Concurrency and Consistency

  1. Concurrent reads on multiple instances – When users on two different instances need to read the same set of blocks.

2. Concurrent reads and writes on different instances - A user intends to read a data block that was recently modified, and the read can be for the current version of the block or for a read-consistent previous version.

3. Concurrent writes on different instances – When the same set of data blocks are modified by different users on different instances.

Cache Coherency

Whether the database is a single-instance stand-alone system or a multi-instance RAC system, maintaining data consistency is a fundamental requirement. If data blocks are already available in the local buffer cache, then they are immediately available for user processes. And, if they are available in another instance within the cluster, they will be transferred into the local buffer cache.

Maintaining the consistency of data blocks in the buffer caches of multiple instances is called ‘cache coherency’. The global cache service (GCS), implemented by a set of Oracle processes requires an instance to acquire cluster-wide data before a block can be modified or read. In this way, cache coherency is ensured and maintained. This resource can be explained in terms of enqueue and/or lock.

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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