A control file tracks the physical components of the database. It is the root file that the database uses to find all the other files used by the database. Because of the importance of the control file, Oracle recommends that the control file be multiplexed, or have multiple identical copies. For databases created with Oracle Database Configuration Assistant (DBCA), two copies of the control file are automatically created and kept synchronized with each other.
If any control file fails, then your database becomes unavailable. If you have a control file copy, however, you can shut down your database and re-create the failed control file from the copy, then restart your database. Another option is to delete the failed control file from the CONTROL_FILES initialization parameter and restart your database using the remaining control files.
See Also:
Oracle Database Concepts for an overview of control files
Oracle Database Administrator's Guide for detailed information about control files