Control File Contents

A control file contains information about the associated database that is required for access by an instance, both at startup and during normal operation. Control file information can be modified only by Oracle; no database administrator or user can edit a control file.

Among other things, a control file contains information such as:

• The database name

• The timestamp of database creation

• The names and locations of associated datafiles and redo log files

• Tablespace information

• Datafile offline ranges

• The log history

• Archived log information

• Backup set and backup piece information

• Backup datafile and redo log information

• Datafile copy information

• The current log sequence number

• Checkpoint information

The database name and timestamp originate at database creation. The database name is taken from either the name specified by the DB_NAME initialization parameter or the name used in the CREATE DATABASE statement.

Each time that a datafile or a redo log file is added to, renamed in, or dropped from the database, the control file is updated to reflect this physical structure change. These changes are recorded so that:

• Oracle can identify the datafiles and redo log files to open during database startup

• Oracle can identify files that are required or available in case database recovery is necessary

Therefore, if you make a change to the physical structure of your database (using ALTER DATABASE statements), then you should immediately make a backup of your control file.

Control files also record information about checkpoints. Every three seconds, the checkpoint process (CKPT) records information in the control file about the checkpoint position in the redo log. This information is used during database recovery to tell Oracle that all redo entries recorded before this point in the redo log group are not necessary for database recovery; they were already written to the datafiles.