control file

The control file is a file that the Oracle maintains to manage the state of the database, and it is the single most important file in the Oracle database. Every database has one control file, but due to its importance, multiple identical copies should maintain. When the database writes to the control file, all the multiplexed copies of the file get written with the same information i.e. identical. The control file is critical to the functioning of the database, and recovery is difficult without access to an updated control file.

The control file contains the names and physical locations, in oracle readable binary format, of the datafiles, redo log files, current log sequence numbers, backup set details, and the all-important system change number (SCN), which indicates the most recent version of committed changes in the database.

Only Oracle can write information to the control file, and the Oracle server process continually updates the control file during the operation of the database.

The control file is important in verifying the integrity of the database and when recovering the database. 

When Oracle instance starts, it consults the control file first, to identify all the datafiles and the redo log files that must be opened for database operations.