Because each node in a Real Application Cluster has its own memory (cache) that is not shared with other nodes, RAC must coordinate the buffer caches of different nodes while minimizing additional disk I/O that could reduce performance.
Cache Fusion is the technology that uses high-speed interconnects to provide cache-to-cache transfers of data blocks between instances in a cluster.
Cache Fusion functionality allows direct memory writes of dirty blocks to alleviate the need to force a disk write and reread (or ping) of the committed blocks. This is not to say that disk writes do not occur; disk writes are still required for cache replacement and when a checkpoint occurs. Cache Fusion addresses the issues involved in concurrency between instances: concurrent reads on multiple nodes, concurrent reads and writes on different nodes, and concurrent writes on different nodes.
Oracle only reads data blocks from disk if they are not already present in the buffer caches of any instance. Because data block writes are deferred, they often contain modifications from multiple transactions. The modified data blocks are written to disk only when a checkpoint occurs. Before I go further, you need to be familiar with a couple of concepts introduced in Oracle 9i RAC: resource modes and resource roles. Because the same data blocks can concurrently exist in multiple instances, there are two identifiers that help to coordinate these blocks:
Global Resource Directory (GRD) is not a database. It is a collection of internal structures and is used to find 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 information about a resource is available in GRD:
Oracle only reads data blocks from disk if they are not already present in the buffer caches of any instance. Because data block writes are deferred, they often contain modifications from multiple transactions. The modified data blocks are written to disk only when a checkpoint occurs. Before I go further, you need to be familiar with a couple of concepts introduced in Oracle 9i RAC: resource modes and resource roles. Because the same data blocks can concurrently exist in multiple instances, there are two identifiers that help to coordinate these blocks:
- Resource mode The modes are null, shared, and exclusive. The block can be held in different modes, depending on whether a resource holder intends to modify data or merely read it.
- Resource role The roles are locally managed and globally managed.
Global Resource Directory (GRD) is not a database. It is a collection of internal structures and is used to find 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 information about a resource is available in GRD:
- Data Block Identifiers (DBA)
- Location of most current versions
- Data blocks modes (N, S, X)
- Data block roles (local or global)
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