Oracle control files

The controlfile is a small binary file that stores some important characteristics of the database:

database name
database unique identifier (DBID)
database creation timestamp
names and locations (filesystem paths) of data files and online redo log files and archived redo log files
information about tablespaces
last checkpoint SCN
(?) history of log switches
(?) the current redo log sequence

Additionally, RMAN uses the control file to store a history of backup metadata.

Purpose

The control file contains the necessary information to open a database such as the location of data files and redo log files)

Thus, when an instance is started up, it first goes through a phase when the control files are opened but the database is still closed.

A control file also contains the information that is required to recover a database (such as checkpoints).

The default location for control files can be found with

select
   default_value
from
   v$parameter
where
   name = 'control_files';

Some dictionary views that are related to control files are:

`v$database`	This view displays some (most, all?) of the information that is stored in the control file.
`v$controlfile`	Shows the physical location of the control files.
`v$controlfile_record_section`	Shows the sections that are stored in a control file.
`v$parameter`	`select value from v$parameter where name = 'control_files'` to get the locations of the control files as specified with the `control_files` init parameter.

Oracle recommends to turn on autobackup on for control files:

configure controlfile autobackup on;

configure controlfile autobackup format for device type disk to 'controlfile_%f';

restore controlfile from autobackup;

The entries of a control file can be dumped to a trace file by setting the following event:

alter session set events 'immediate trace name controlf level 10';

Alternatively, it can also be dumped with oradebug dump controlf.