Cache fusion in 10g

Cache Fusion addresses several types of concurrency

* Concurrent Reads on Multiple Nodes
* Concurrent Reads and Writes on Different Nodes
* Concurrent Writes on Different Nodes

Concurrent Reads on Multiple Nodes

Concurrent reads on multiple nodes occur when two instances need to read the same data block. Real Application Clusters resolves this situation without synchronization because multiple instances can share data blocks for read access without cache coherency conflicts.
Concurrent Reads and Writes on Different Nodes

A read request from an instance for a block that was modified by another instance and not yet written to disk can be a request for either the current version of the block or for a read-consistent version. In either case, the Global Cache Service Processes (LMSn) transfer the block from the holding instance's cache to the requesting instance's cache over the interconnect.
Concurrent Writes on Different Nodes

Concurrent writes on different nodes occur when the same data block is modified frequently by different instances. In such cases, the holding instance completes its work on the data block after receiving a request for the block. The GCS then converts the resources on the block to be globally managed and the LMSn processes transfer a copy of the block to the cache of the requesting instance. The main features of this processing are:

* The Global Cache Service (GCS) tracks a each version of a data block, and each version is referred to as a past image (PI). In the event of a failure, Oracle can reconstruct the current version of a block by using the information in a PI.
* The cache-to-cache data transfer is done through the high speed IPC interconnect, thus eliminating disk I/O.
* Cache Fusion limits the number of context switches because of the reduced sequence of round trip messages. Reducing the number of context switches enables greater cache coherency protocol efficiency. The database writer (DBWn) processes are not involved in Cache Fusion block transfers.