start，mount,open

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