Oracle Database - Instance (Database Start-up)


When you start a database, an instance is created.

An instance is comprised of:

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.

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.

How to


A instance may be mount and open a single database at any point of time. See the Startup command of SQLPlus

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.



select status from v$instance;

You start up an instance by mounting and opening the database. Doing so makes the database available for any valid user to connect to and perform typical data access operations.

Status Mode Startup Option Description Tips
NOMOUNT - NOMOUNT Does not allow access to the database usually done only for database creation or the re-creation of control files.
Mount - MOUNT Allows DBA activities, but does not allow general access to the database
Open Restricted mode RESTRICT Allow access for database administrators only
Open Unrestricted Allow access to all users
FORCE Forcing an Instance to Start
OPEN RECOVER start an instance, mount a database to the instance, and have the recovery process automatically start



The parameters of an instance

With SYS_CONTEXT in 10g and higher, you can get almost ALL instances attributes that are available in v$instance but without needed permission.

from dual;


One Instance for Each Database

Multiple instances on different computers that share a single physical database

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.

Documentation / Reference

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