When starting the database instance, select the state in which it starts. The following scenarios describe different stages of starting up an instance.
Starting Up an Oracle Database Instance: NOMOUNT
An instance is typically started only in NOMOUNT mode during database creation,
during re-creation of control files, or during certain backup and recovery scenarios.
Starting an instance includes the following tasks:
Searching $ORACLE_HOME/dbs for a file of a particular name in this sequence:
1. Search for spfile<SID>.ora.
2. If spfile<SID>.ora is not found, search for spfile.ora.
3. If spfile.ora is not found, search for init<SID>.ora.
This is the file that contains initialization parameters for the instance. Specifying the PFILE parameter with STARTUP overrides the default behavior.
Allocating the SGA
Starting the background processes
Opening the alert_<SID>.log file and the trace files
Note: SID is the system ID, which identifies the instance (for example, ORCL).
Starting Up an Oracle Database Instance: MOUNT
Mounting a database includes the following tasks:
- Associating a database with a previously started instance
- Locating and opening the control files specified in the parameter file
- Reading the control files to obtain the names and statuses of the data files and online redo log files (However, no checks are performed to verify the existence of the data files and online redo log files at this time.)
To perform specific maintenance operations, start an instance and mount a database, but do not open the database.
For example, the database must be mounted but must not be opened during the following tasks:
- Renaming data files (Data files for an offline tablespace can be renamed when the database is open.)
- Enabling and disabling online redo log file archiving options
- Performing full database recovery
Note: A database may be left in MOUNT mode even though an OPEN request has been made. This may be because the database needs to be recovered in some way. If recovery is performed while in the MOUNT state, the redo logs are open for reads and the data files are open as well to read the blocks needing recovery and to write blocks if required during recovery.
Starting Up an Oracle Database Instance: OPEN
A normal database operation means that an instance is started and the database is mounted and opened. With a normal database operation, any valid user can connect to the database and perform typical data access operations.
Opening the database includes the following tasks:
- Opening the data files
- Opening the online redo log files
If any of the data files or online redo log files are not present when you attempt to open the database, the Oracle server returns an error.
During this final stage, the Oracle server verifies that all data files and online redo log files can be opened, and checks the consistency of the database. If necessary, the System Monitor (SMON) background process initiates instance recovery.
You can start up a database instance in restricted mode so that it is available to users with administrative privileges only. To start an instance in restricted mode, select the “Restrict access to database” option on the Advanced Startup Options page.
