Oracle in a Nutshell

Control Files

The control file contains a list of all the other files that make up the database, such as the datafiles and redo log files. It also contains key information about the contents and state of the database, such as:

• The name of the database

• When the database was created

• The current state of the datafiles: whether they need recovery, are in a read-only state, and so on

• Whether the database closed cleanly the last time it was shut down

• The time period covered by each archived redo log

• What backups have been performed for the database

Prior to Oracle8, control files were typically under a megabyte in size. With Oracle8, there is more information in the control file, such as the details of database backups. The control files in Oracle8 and beyond can easily grow to the 10 MB range or beyond. The size of a control file is influenced by a number of initialization parameters, including MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES, and MAXINSTANCES.

You can have your Oracle instance maintain multiple copies of control files. Although you can potentially rebuild a control file if it is damaged or deleted, this process takes time and in some scenarios you cannot rebuild the control file to its correct state. You cannot run an Oracle database without a control file, so having multiple copies of your control file can be an important safety option. You use the initialization parameter CONTROL_FILES to list the locations of multiple copies of the control file.