Oracle Database - Instance (Database Start-up)

When you start a database, an instance is created.

An instance is comprised of:

• a memory area called the System Global Area (SGA)
• and Oracle background processes.

The memory and processes of an instance efficiently manage the associated database's data and serve the database users.

A database has at least one instance. Then you can have many instance of the same database.

A instance may mount and open a single database at any point of time.

SQL> STARTUP
ORACLE instance started.

Total System Global Area  619360256 bytes
Fixed Size                  1338280 bytes
Variable Size             377488472 bytes
Database Buffers          234881024 bytes
Redo Buffers                5652480 bytes
Database mounted.
Database opened.

An instance also manages other services, such as Oracle XML DB

You can connect to any instance to access information within a cluster database.

Instances are identified by an instance name. The instance name is specified by the INSTANCE_NAME initialization parameter.

Some hardware architectures allow multiple computers to share access to data, software, or peripheral devices. Oracle Real Application Clusters (Oracle RAC) (Oracle RAC) can take advantage of such architecture by running multiple instances on different computers that share a single physical database.