Here's what Oracle's Concepts Guide has to say:
A system global area (SGA) is a group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users. Consequently, the SGA is sometimes referred to as the shared global area.
An SGA and Oracle processes constitute an Oracle instance. Oracle automatically allocates memory for an SGA when you start an instance and the operating system reclaims the memory when you shut down the instance. Each instance has its own SGA.
The SGA is read-write; all users connected to a multiple-process database instance may read information contained within the instance's SGA, and several processes write to the SGA during execution of Oracle.
The SGA contains the following data structures:
- The database buffer cache
- The redo log buffer
- The shared pool
- The large pool (optional)
- The data dictionary cache
- Other miscellaneous information
For more great info see the Oracle Concepts Guide.
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This was first published in March 2003