THE CONTROL FILE

THE CONTROL FILE

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Introduction:

A control file is the most critical component of a database, as it stores important information that is not available in any other file. It is a small binary file that records the physical structure of a database and defines its current state. A database is mounted using a control file. Therefore, at least one control file must be present in a database. However, the minimum and maximum recommended number of control files is two, and eight respectively. Furthermore, a control file may belong to only one database.

A control file contains the following information:

• The name of the database
• The name, location, status and size of data files and redo log files
• Information used to recover the database in case of a disk failure or user error
• Database creation date
• Database ID, which is unique for every database
• Archive log history
• Tablespaces and data file records
• Current archive log mode
• Log records (SCN range)
• RMAN backup and copy records
• Block corruption information
• The database creation timestamp
• The name of the associated tablespaces
• The most recent checkpoint information
• The beginning and ending of the undo segments
• Recovery managers back up information

A control file is created at the time of database creation at a location that is specified in the control file parameters in the parameter file of the database. If a control file is corrupted, then an issue will arise regarding how to recover the database. For this reason, most production databases multiplex their control files to enable Oracle to write multiple copies of a control file to many locations, all on different disks. Oracle 10g use the CKPT background process to automatically update each of the control files as needed, keeping the contents of all copies of the control file always synchronized.



The location of the control files is specified through the control files init parameter. The control files have to be made available all the time because without them, the database cannot operate, as the control files keep track of all the data files.



Criticality of the control file:

When the control files are lost or corrupted, then it is very difficult to do the recovery operations, as the database cannot be mounted for the recovery purposes.

The re-creation of the control files is required in the following mentioned circumstances:

• Loss of all the copies of the control files
• The requirement of change in the name of the database
• The requirement to change the maximum number of redo log file groups, redo log file members, archived redo log files, and data files

Oracle recommends that there should be a full back up of all the database files before re-creating the control files.

Following syntax can be used to re-create the control file:

ALTER DATABASE BACK UP CONTROL FILE TO TRACE;.

This will generate a new SQL script that may be used to re-create the control files.



Note: The control files are, however, not limited to the above-mentioned information.

Conclusion:

It will not be incorrect to say that a control file is the uttermost crucial file of a database. In its absence a database is unable to operate. Therefore, control files must always be multiplexed.

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