Oracle7 Server Concepts

Control Files

The control file of a database is a small binary file necessary for the database to start and operate successfully. A control file is updated continuously by Oracle during database use, so it must be available for writing whenever the database is open. If for some reason the control file is not accessible, the database will not function properly.

Each control file is associated with only one Oracle database.

Control File Contents

A control file contains information about the associated database that is required for the database to be accessed by an instance, both at startup and during normal operation. A control file's information can be modified only by Oracle; no database administrator or end-user can edit a database's 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 databases and online redo log files

the current log sequence number

checkpoint information

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

Each time that a datafile or an online 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 online 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 your database's physical structure, you should immediately make a backup of your control file. See "Control File Backups"

for information about backing up a database's control file.

Control files also record information about checkpoints. When a checkpoint starts, the control file records information to remember the next entry that must be entered into the online redo log. This information is used during database recovery to tell Oracle that all redo entries recorded before this point in the online redo log group are not necessary for database recovery; they were already written to the datafiles. See "Checkpoints" .

Multiplexed Control Files

As with online redo log files, Oracle allows multiple, identical control files to be open concurrently and written for the same database. By storing multiple control files for a single database on different disks, you can safeguard against a single point of failure with respect to control files. If a single disk that contained a control file crashes, the current instance fails when Oracle attempts to access the damaged control file. However, other copies of the current control file are available on different disks, so an instance can be restarted easily without the need for database recovery.

The permanent loss of all copies of a database's control file is a serious problem to safeguard against. If all control files of a database are permanently lost during operation (several disks fail), the instance is aborted and media recovery is required. Even so, media recovery is not straightforward if an older backup of a control file must be used because a current copy is not available. Therefore, it is strongly recommended that multiplexed control files be used with each database, with each copy stored on a different physical disk.