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

GCS Resource Modes and Roles

Basically, the concept of role supplements the access mode characteristic. A typical data block has both of these characteristics.

Concept of Past Image

The past image concept was introduced in the RAC version of Oracle 9i to maintain data integrity. In an Oracle database, a typical data block is not written to the disk immediately, even after it is dirtied. When the same dirty data block is requested by another instance for write or read purposes, an image of the block is created at the owning instance, and only that block is shipped to the requesting instance. This backup image of the block is called the past image (PI) and is kept in memory. In the event of failure, Oracle can reconstruct the current version of the block by reading PIs. It is also possible to have more than one past image in the memory depending on how many times the data block was requested in the dirty stage.

A past image copy of the data block is different from a CR block, which is needed for reconstructing a read-consistent image. A CR version of a block represents a consistent snapshot of the data at a point in time. It is constructed by applying information from the undo/rollback segments. The PI image copy helps the recovery process and aids in maintaining data integrity.

For example, suppose user A of instance 1 has updated row 2 on block 5. Later, user B of instance 2 intends to update row 6 on the same block 5. The GCS transfers block 5 from instance A to instance B. At this point, the past image (PI) for block 5 is created on instance A.

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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