Oracle手册上对于SGA(System Global Area)的定义:

The SGA is a read/write memory area that, along with the Oracle background processes, form a database instance. All server processes that execute on behalf of users can read information in the instance SGA. Several processes write to the SGA during database operation.

Note: The server and background processes do not reside within the SGA, but exist in a separate memory space.

Each database instance has its own SGA. Oracle Database automatically allocates memory for an SGA at instance startup and reclaims the memory at instance shutdown.

SGAOracle后台进程一起组成了数据库的Instance。每个Instance拥有自己的SGAInstance启动时会自动分配SGA,退出时会回收SGASGA包含下列组成部分:

✓ Shared pool
✓ Database buffer cache
✓ Redo log buffer
✓ Large pool
✓ Java pool
✓ Streams pool

参考资料:
Oracle 12c For Dummies
Overview of the System Global Area (SGA)