Oracle Database Tips by Donald Burleson

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

The GES and GCS together maintain a global resource directory (GRD) to record information about resources and enqueues. The GRD remains in the memory and is stored on all the instances. Each instance manages a portion of the directory. The distributed nature of the GRD is a key point for the fault tolerance of RAC. The GRD is an internal database that records and stores the current status of the data blocks. Whenever a block is transferred out of a local cache to another instance's cache, the GRD is updated. The following resource information is available in GRD:

• Data Block Addresses (DBA). This is the address of the block being modified.
   
• Location of most current version of the data block. This exists only if multiple nodes share the data block.
   
• Modes of the data blocks ((N)Null, (S)Shared, (X)Exclusive ).
   
• The Roles of the data blocks (local or global). This indicates the role in which the data block is being held by the instance.
   
• SCN - System Change Number.
   
• Image of the Data Block - it could be past image or current image. Current image represents the copy of the block held by the current instance. Past image represents the global dirty data block image maintained in the cache.