Cache Fusion is the process of combining cache from each node/instance participating in the RAC, but it is not a physical memory component.
Every instance of the RAC database has its own local buffer cache which performs the usual cache functionality for that instance. There could be occasions when a transaction on instance A needs to access a data block which is being owned/locked by other instance B. In such cases, the instance A will request instance B for that data block and hence accesses the block through interconnect mechanism. This concept is known as Cache Fusion where one instance can work on or access a data block in other instance cache via the high speed interconnect.
Global Cache Service [GCS]
GCS is the heart of Cache Fusion concept. It is through GCS that data integrity in RAC is maintained when more than one instance need a particular data block. Instances look up to the GCS for fulfilling their data block needs.
GCS is responsible for,
– Tracking the data block
– Accepting the data block request from instances
– Informing the holding instance to release the lock on the data block or ship a CR image
– Coordinating the shipping of data blocks through the interconnect
GCS is available in the form of background process called LMS.