Understanding the RAC Cache Fusion architecture, components and internals is the key to successfully tuned RAC databases. Let's briefly review the concepts of RAC prior to discussing the tuning issues. Each instance has a buffer cache in its System Global Area (SGA). Using Cache Fusion, Oracle RAC environments logically combine each instance's buffer cache to enable the instances to process data if the data resides on a logically combined, single cache. The SGA size requirements for Oracle RAC are greater than the SGA requirements for single-instance Oracle databases due to Cache Fusion.
To ensure that each Oracle RAC database instance obtains the block that it requires and to satisfy a query or transaction, Oracle RAC instances use two processes—the Global Cache Service (GCS) and the Global Enqueue Service (GES).
The GCS and GES maintain records of the statuses of each data file and each cached block using a Global Resource Directory. The GRD...