Oracle Tips by Burleson

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

Cache Fusion

A RAC system equipped with low-latency and high speed inter-connect technology enables the buffer cache of each node in the cluster to fuse and form into a single virtual ‘global cache’, hence the term cache fusion. The cache fusion architecture creates a shared-cache and provides a single cache image or view to the applications. Internals are transparent to the applications.

From a functional viewpoint, an instance in a RAC system is equivalent to a single instance of Oracle. It has all the bells and whistles of a single instance, which the DBA(s) understand very well. The extension of multiple cache buffers into a single, fused global cache improves scalability, reliability, and availability.

While cache fusion provides Oracle users with an expanded database cache for queries and updates of I/O operations, the improved performance depends greatly on the efficiency of the inter-node message passing mechanism that handles the data block transfers.

Evolution of Cache Fusion

Before looking deeper into the implementation of cache fusion in Oracle 9i RAC, let’s have a look at the implementation in the 8i release. Oracle Release 8i (Oracle Parallel Server) introduced the initial phase of cache fusion. The data blocks were transferred from the SGA of one instance to the SGA of another instance without the need to write the blocks to disk. This was aimed at reducing the ‘ping’ overhead of data blocks. However, the partial implementation of cache fusion in 8i could help only in certain conditions, as indicated in Table 7.1

Requesting Instance  Dirty Block exists in Holding Instance    Cache Coherency Method
Instance     Holding
---------    --------  -------------------------------------      --------------------------
For Read    Read     Yes                                                        Cache Fusion