Multiplexing Control File

A Control file is a must file for a Oracle Database to start-up and function. The control file includes:

1. The Database name
2. Names and locations of associated data files and redo log files
3. The time stamps of the database creation
4. The current log sequence number
5. Checkpoint information

The control file should be available during startup and it has to be available at all time for writing. Without control file,the database can’t be mounted. It holds information about Oracle Database.
Some one asked me, can you start a database without control file? Yes, that’s possible but it is not an easy task.

Multiplexing:  

The term multiplexing is frequently used in Telecom Industries which means mirroring in plain English. It is an act of sending signals to multiple directions at a same time. How is this relevant to Oracle database? Oracle db has the ability to write to more than one files (max 8 files - 11g) at a given time. This is done to preserve the copy of a file.

Multiplexing of control file minimized the risk of losing a copy during single disk or media failure. I can’t stress enough, how valuable the control file is? We need to do whatever we can to preserve the control file. It is recommended that you have at-least 3 copies of control files on different medias. The idea her is not to store all the eggs in a same basket!

Check if your database control file is Multiplexed or not:

show parameter control_file;

Or you may use these queries!

SELECT * 
FROM   v$parameter 
WHERE  name LIKE '%control%'; 

SELECT name 
FROM   v$controlfile; 

How do you multiplex control file in Oracle 11g?

1. Go to the directory where the control file is located (show parameter control_file; -- for file path dir.)
2. Log into SQLPlus and shutdown the database. (Why?) We don't want the updating happening in a control file while making a copy of control file.
3. Make few copies of Oracle Control File. (control02.ctl and control03.ctl).You now have three control files. Store them in different storage media.
4. Update the PFILE with two new control files. (show parameter pfile; --for pfile/spile location) for location of these new control files.
5. Create spfile from pfile;
6. Startup database. 
7. You are done with Multiplexing!!
8. Verify the new control files are under show parameter control_file;

Similarly, you can  do multiplexing of  re-do logs. I will let you figure it out on how you do for re-do logs.