Oracle Security

The Instance and the Database: Starting an Oracle Database

Many people use the terms instance and database interchangeably, but the instance and the database are actually separate entities. Let’s look at what happens when the database is started from the Server Manager utility (svrmgrl, svrmgrm, or svrmgr30 for Oracle8 on Windows NT). The startup process follows:

• The background processes are started.

• The System Global Area (SGA) is allocated in memory.

• The background processes open the various files.

• The database is ready for use.

During the startup, messages are displayed that notify you of what is happening. Here is a sample startup sequence:

SVRMGR>  startup
ORACLE instance started.
Total System Global Area      11865072 bytes
Fixed Size                       33708 bytes
Variable Size                 10672196 bytes
Database Buffers               1126400 bytes
Redo Buffers                     32768 bytes
Database mounted.
Database opened.

You can see that the instance is started before the files that constitute the database are opened. The instance consists of the background processes and the SGA. The SGA totals are listed before the database is started but after the “ORACLE instance started” message. The database is the collection of logical objects and physical files necessary to support the system, and the database system is the instance, SGA, and files. Figure 2.1 shows the components of the database system after the database has been started.

Figure 2-1. Components ...