The basic memory structures associated with an Oracle instance include the following:

•System Global Area (SGA): Shared by all server and background processes

•Program Global Area (PGA): Private to each server and background process. There is one PGA for each process.

The SGA is a memory area that contains data and control information for the instance.

The SGA includes the following data structures:

•Database buffer cache: Caches blocks of data retrieved from the database

•Redo log buffer: Caches redo information (used for instance recovery) until it can be written to the physical redo log files stored on the disk

•Shared pool: Caches various constructs that can be shared among users

•Large pool: Is an optional area that provides large memory allocations for certain large processes, such as Oracle backup and recovery operations, and I/O server processes

•Java pool: Is used for all session-specific Java code and data within the Java Virtual Machine (JVM)

•Streams pool: Is used by Oracle Streams

When you start the instance by using Enterprise Manager or SQL*Plus, the amount of memory allocated for the SGA is displayed.

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SGA和PGA

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