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Oracle Database Administration: The Essential Refe by Brian Laskey, David C. Kreines

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Control files

Every Oracle instance must have one or more control files. The control file is a binary file that is critical to Oracle, but is not directly readable by a user, nor is it editable by a text editor. The control file can be thought of as a software “bootstrap” file; it contains information that Oracle requires to start. Information stored in the control file includes:

  • Names and locations of data files

  • Names and locations of redo log files

  • Information on the status of archived log files

  • The current redo log sequence number

  • Redo log information required for recovery

  • Backup history (Oracle8 only)

  • Timestamp information on the instance creation and startup/shutdown

  • Essential parameters specified at database creation (e.g., MAXDATAFILES)

The information stored in the control file is so critical that if the control file is lost or damaged, the only options available for recovery are either to create a new control file (assuming that the DBA has access to all pertinent information required) or to rebuild the database and restore from a backup. Because of the critical nature of the control file, Oracle allows the DBA to maintain multiple mirrored control files, as specified by the CONTROL_FILES parameter in the INIT.ORA file. For example, the following line from INIT.ORA specifies two control files:

CONTROL_FILES = (/disk00/oracle/control01.ctl,/disk02/oracle/control02.ctl)

Tip

We strongly recommend that you maintain multiple mirrored control files on separate disks in case disk failure ...

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