Oracle Database Tips by Donald Burleson

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

The instance is the structure or entity with which application users connect. The SGA is a group of shared memory structures that contain data and control information for the database instance. Oracle allocates memory for an SGA system whenever the instance is started. Multiple instances can be associated with a database in a RAC system, and each instance has its own SGA. The SGA contains five main areas.

1. The fixed area.
    
2. The variable area.
    
3. The database buffer cache.
    
4. The log buffer.
    
5. The resource directory for a RAC system.

The variable part of the SGA is made up of a large pool and a shared pool. All memory in the large pool is dynamically allocated, whereas the shared pool contains both dynamically managed memory and a permanent memory. The database buffer cache is where database block copies are held for processing. All user processes concurrently connected to the instance share access to the database buffer cache. There are many groups of buffers within the SGA.