System Global Area (SGA)
The System Global Area (SGA) is the most important memory structure of Oracle and consists of several different components. The SGA is a shared memory structure, and contains all memory-resident data and control information for a single Oracle instance (each instance must have its own SGA). Because the SGA data is shared among all users of the instance, it is sometimes called the Shared Global Area. Note that Oracle processes may both read from and write to the SGA.
Memory for the SGA is allocated when the instance is started and released when the instance is shut down. The size of the SGA is controlled primarily by various INIT.ORA parameters.
The SGA contains several components, described in the following sections.
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