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  Oracle Database Tips by Donald Burleson

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

GCS Resource Modes and Roles

Null (N) mode - Holding a resource at this level conveys that there are no access rights. Null mode is usually held as a placeholder, even if the resource is not actively used. [Steve Adams]

Shared (S) mode
- When a resource is held at this level, it will ensure that the data block is not modified by another session, but will allow concurrent shared access.

Exclusive (X) mode
- This level grants the holding process exclusive access. Other processes cannot write to the resource. It may have consistent read blocks.

The resource mode is an important mechanism to maintain data integrity and avoid data corruption issues. GCS resources are specified to have global roles or local roles. These roles are mutually exclusive.

When a block is first read into the cache of an instance and other instances have not read the same block, then the block is said to be locally managed and is therefore assigned a local role.

After the block has been modified by the local instance and transmitted to another instance, it is considered to be globally managed, and is therefore assigned a global role.

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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