21-01-2006

Control file is one of the most important file of Oracle database.

If control file is lost then entire database is lost. Thus back up of this file is very important and hence Oracle allows you to replicate this file i.e. make its mirror images. Oracle recommends you to store these mirror images on different hard-disks.

This file maintains synchronization information about about all the database files. The synchronization is very important from point of view of recovery of database. At the time of start up, Oracle checks the integrity of all the files from the control file.

The synchronization information is maintained in terms of SCN (System Change Number), CPC (Check Pointer Counter) and LSN (Log sequence Number). Control file contains the copies of headers of all data files and redo log files. Oracle is constantly maintains the headers of database files and redo log files and their copies in control file synchronously. If there is any mismatch between them, Oracle knows that recovery is needed to be applied in some form or the other. Apart from synchronization information control file also maintains some of the important database parameters exclusive to the control file like maxdatafiles, maxlog files, maxlog member, maxlog history, characteristic etc. It is possibe to modify the parameters expect the character set parameter. However for modifying the parameter control file is needed to be recreated with the new parameters.

*: Characteristic is similar to block size.

System Change Number is a no given to every committed transaction.

Check Point Counter indicates the number of times a file is changed.

Control file automatically get created at the time of creation of database. It also contains the name of database. Control file is fixed size file and it is in binary format. This file should never be edited or opened. After the version of Oracle 8i onwards control file also maintains the record of back and recovery taken by RMAN (Recovery Manager).