How an Instance Is Started
When Oracle Database
starts an instance, it performs the following basic steps:
1.
Searches for a server
parameter file in a
platform-specific default location and, if not found, for a text initialization
parameter file (specifying STARTUP with
the SPFILE or PFILE parameters
overrides the default behavior)
2.
Reads the parameter file to determine the values of initialization parameters
3.
Allocates the SGA based on the initialization parameter settings
4.
Starts the Oracle background processes
5.
Opens the alert log and
trace files and writes all explicit parameter settings to the alert log in
valid parameter syntax
At this stage, no database is associated with the instance. Scenarios that require a NOMOUNT state
include database creation and certain backup
and recovery operations.
See Also:
Oracle Database Administrator's Guide to
learn how to manage initialization parameters using a server parameter file
How a Database Is Mounted
The
instance mounts a database to associate the database with this instance.To mount the database, the
instance obtains the names of the database control files specified in the CONTROL_FILES initialization
parameter and opens the files.
Oracle Database reads the control files to find the names of the data files and the online redo log files that it will attempt to access when opening the database.
In a mounted database,
the database is closed and accessible only to database administrators. Administrators can keep the database closed while completing specific maintenance operations. However, the database is not available for normal operations.
If
Oracle Database allows multiple instances to mount the same database concurrently,then the CLUSTER_DATABASEinitialization
parameter setting can make the database available to multiple instances. Database behavior depends on the setting(ZYD)
· If CLUSTER_DATABASE is false (default)
for the first instance that mounts a database, then only this instance can mount the database.
· If CLUSTER_DATABASE is true for
the first instance, then other instances can mount the database if their
CLUSTER_DATABASE parameter settings are set to true.
The number of instances that can mount the database is subject to a predetermined maximum specified when creating the database.
See Also:
· Oracle Database Administrator's
Guide to learn how to mount a database
· Oracle Real Application Clusters Administration
and Deployment Guide for more information about the use of multiple instances with a single database
How a Database Is Opened
Opening a mounted database
makes it available for normal database operations.Any valid user can connect to an open database and access its information.
Usually, a database administrator opens the database to make it available for general use.
When you open the database, Oracle Database performs the following actions:（ZYD）
· Opens the online data files in tablespaces other than undo tablespaces
If a tablespace was offline when the database was previously shut down (see "Online
and Offline Tablespaces"), then the tablespace and its corresponding data files will be offline when the database reopens.
· Acquires an undo tablespace
If multiple undo tablespaces exists, then the UNDO_TABLESPACE initialization
parameter designates the undo tablespace to use. If this parameter is not set, then the first available undo tablespace is chosen.
· Opens the online redo log files