Oracle in a Nutshell

The Components of an Instance

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 described in the next section, “Memory Structures for an Instance.” 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 started. The only way you could change the size of the SGA or any of its components was to change the appropriate initialization parameters and stop and restart the instance. With Oracle9i, you can now change the size of the SGA or its components while the Oracle instance is still running.

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.

Other physical files can be considered as part of the instance as well:

Instance initialization file

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, what database the instance actually provides access to, and so on. You can alter many of these parameters dynamically at either the systemwide or session-specific level. ...