An Oracle instance can be defined as an area of shared memory and a collection of background processes. The area of shared memory for an instance is called the System Global Area, or SGA. The SGA is not really one large undifferentiated section of memory—it’s made up of various components that we’ll examine in the next section, Section 2.3.1. All the processes of an instance—system processes and user processes—share the SGA.
Prior to Oracle9i, the size of the SGA was set when the Oracle instance was started. The only way to change the size of the SGA or any of its components was to change the initialization parameter and then stop and restart the instance. With Oracle9i, you can change the size of the SGA or its components while the Oracle instance is still running. Oracle9i introduces the concept of the granule, which is the smallest amount of memory that you can add to or subtract from the SGA.
The background processes interact with the operating system and each other to manage the memory structures for the instance. These processes also manage the actual database on disk and perform general housekeeping for the instance.
There are other physical files that you can consider as part of the instance as well:
The initialization file contains a variety of parameters that configure how the instance will operate: how much memory it will use, how many users it will allow to connect, to which database the instance actually ...