Oracle7 Server Concepts
An Oracle Instance
Regardless of the type of computer executing Oracle and the particular memory and process options being used, every running Oracle database is associated with an Oracle instance. Every time a database is started on a database server, Oracle allocates a memory area called the System Global Area (SGA) and starts one or more Oracle processes. The combination of the SGA and the Oracle processes is called an Oracle database instance. The memory and processes of an instance work to manage the database's data efficiently and serve the one or multiple users of the associated database. Figure 9 - 1 shows an Oracle instance.
Figure 9 - 1. An Oracle Instance
Oracle starts an instance, then mounts a database to the instance. Multiple instances can execute concurrently on the same machine, each accessing its own physical database. In clustered and massively parallel systems, the Oracle Parallel Server allows a single database to mount multiple instances. When you use Trusted Oracle in OS MAC mode, a single instance can mount multiple databases.
The remaining sections of this chapter explain the memory and process structures and configurations associated with an Oracle instance. See Oracle7 Parallel Server Concepts & Administration for more information about the Oracle Parallel Server. See the Trusted Oracle7 Server Administrator's Guide for more information about Trusted Oracle.