Oracle7 Server Concepts
Database and Instance Shutdown
There are three steps to shutting down an instance and the database to which it is connected:
2. Dismount the database.
3. Shut down the instance.
Oracle automatically performs all three steps when an instance is shut down.
Closing a Database
The first step of database shutdown is closing the database. When you close a database, Oracle writes all database data and recovery data in the SGA to the datafiles and redo log files, respectively. After this operation, Oracle closes all online datafiles and online redo log files. Any offline datafiles of any offline tablespaces will have been closed already. When you subsequently reopen the database, the tablespace that was offline and its datafiles remain offline and closed, respectively. The control files remain open after a database is closed but still mounted.
Closing the Database by Aborting the Instance
In rare emergency situations, you can abort the instance of an open database to close and completely shut down the database instantaneously. This process is fast because the operation of writing all data in the buffers of the SGA to the datafiles and redo log files is skipped. The subsequent reopening of the database requires instance recovery, which Oracle performs automatically.
Note: If a system crash or power failure occurs while the database is open, the instance is, in effect, "aborted", and instance recovery is performed when the database is reopened.
Dismounting a Database
The second step accomplished during database shutdown is dismounting or disassociating the database from an instance. After you dismount a database, only an instance remains in the memory of your computer.
After a database is dismounted, Oracle closes the control files of the database.
Shutting Down an Instance
The final step in database shutdown is shutting down the instance. When you shut down an instance, the SGA is removed from memory and the background processes are terminated.
Abnormal Instance Shutdown
In unusual circumstances, shutdown of an instance might not occur cleanly; all memory structures might not be removed from memory or one of the background processes might not be killed. When remnants of previous instances exist, subsequent instance startup most likely will fail. To handle this problem, the database administrator can force the new instance to start up by first removing the remnants of the previous instance and then starting a new instance, or by issuing a SHUTDOWN ABORT statement.