Multiplexing Control Files

To protect the database from a single point of failure occurring in the control file, it is strongly recommended by Oracle that the control file be multiplexed and each copy stored to a different writable disk. If the primary control file is for any reason lost, the database can continue using the multiplexed version, and the instance and database can be restarted (after alteration of the primary control file to one of the backup locations and restarting the instance and database) without the necessity of database recovery. Control files can be multiplexed up to a maximum of eight times. The Oracle server is responsible for creating and maintaining all files listed in the CONTROL_FILES parameter when the instance is started.

Multiple control files can be created when you create the database by setting the CONTROL_FILES initialization parameter to the comma-delimited list of control file location and names.

 CONTROL_FILES='/mydatabases/mydb1/control01.ctl', '/mydatabases2/mydb1/control02.ctl' 

After the database has been created, you can add additional control files. The steps to defining the additional control files differ depending on whether you are using the PFILE or the SPFILE to initialize the database instance.

Multiplexing Using SPFILE

If you are using the server parameter file (SPFILE) to start the instance, follow these steps:

1.	First alter the SPFILE using SQL: ALTER SYSTEM SET CONTROL FILES = 'd:\mydatabase\mydb1\control01.ctl', 'e:\mydatabase\mydb1\control02.ctl' SCOPE=SPFILE;
2.	Shut down the database either in normal mode, or immediate mode: shutdown immediate
3.	Create the new control files using your operating system's copy command.
4.	Restart the database: startup mydb1

You now have multiplexed the control files.

Multiplexing Using PFILE

If you are using the PFILE (initSID.ora), do the following:

1.	Add the new control filename to the initSID.ora file: CONTROL_FILES = ('/mydatabases/mydb1/control01.ctl', '/mydatabases2/mydb1/control02.ctl')
2.	Shut down the database in either normal or immediate manner: shutdown immediate
3.	Copy the existing control file to the new name and location using your operating system's copy command.
4.	Start up and open the database, and you have multiplexed your database.

The following describes the behavior of multiplexed control files:

• Oracle writes to all the files designated in the list for the initialization parameter CONTROL_FILES in the database's initialization parameter file.

• The first file listed in the CONTROL_FILES parameter is the only file read by the Oracle database server during database operation.

• If, however, any of the control files become unavailable during database operation, the instance becomes inoperable and should be closed, often resulting in the necessity of shutdown abort. It can, however, be reopened with one of the multiplexed or the other duplexed version of the control file with the alteration of the PFILE to point to the backup's location. If you have only one control file, you would need to recover the database to reopen it for operation.