Recovering a control file using a current backup copy or a mutiplexed copy

(1) One of the control files specified in CONTROL_FILES parameter is corrupted.
(2) One of the control files is inaccessible due to a media failure.

Even though only one of the multiplexed control files is corrupted or inaccessible, the database can't start up.

(1) When a control file is corrupted
      - Shutdown the instance.
      - Use O/S copy command to overwrite the bad control file with a good copy
          $> cp  /u01/oracle/prod/control01.ctl   /u03/oracle/prod/control3.ctl
      - Start up DB.

(2) When a control file is inaccessible due to a media failure
      - Shutdown the instance.
      - Use O/S copy command to copy the current copy of the control file to a new accessible location
          $> cp /u01/oracle/prod/control01.ctl   /u04/oracle/prod/control03.ctl
      - Edit the Control_files parameter to replace the bad location with the new location
            CONTROL_FILES - (/u01/oracle/prod/control01.ctl,
                                                /u02/oracle/prod/control02.ctl,
                                                /u04/oracle/prod/control03.ctl)
      - Start up the database

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** If we use multiplexed control files, we can just remove the bad control file from the CONTROL_FILES parameter. And we can just start up the database quickly for the db users.  We can then re-construct the bad control file and restart the DB at later time when the database is not busy.

** If any of the multiplexed control file is no longer available or appropriate, we can just remove the control file from the CONTROL_FILES parameter. But we need to make sure that there are always at least 2  control files.