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.
SGA和Oracle后台进程一起组成了数据库的Instance。每个Instance拥有自己的SGA，Instance启动时会自动分配SGA，退出时会回收SGA。SGA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)。