How to Manage Control Files in Oracle

You can create and manage database structures and storage components.

• Managing Control Files
• Managing the Redo Log
• Managing Archived Redo Log Files
• Managing Tablespaces
• Managing Data Files and Temp Files
• Transporting Data
• Managing Undo
• Using Oracle Managed Files

1.Managing Control Files

1.1 What Is a Control File?

Every Oracle Database has a control file, which is a small binary file that records the physical structure of the database.

The control file includes:

• The database name

• Names and locations of associated data files and redo log files

• The timestamp of the database creation

• The current log sequence number

• Checkpoint information

1.2 Guidelines for Control Files

1.2.1 Provide File Names for the Control Files

If you do not specify files for CONTROL_FILES before database creation:

• If you are not using Oracle Managed Files, then the database creates a control file and uses a default file name. The default name is operating system specific.

• If you are using Oracle Managed Files, then the initialization parameters you set to enable that feature determine the name and location of the control files.

• If you are using Oracle Automatic Storage Management (Oracle ASM), you can place incomplete Oracle ASM file names in the DB_CREATE_FILE_DEST and DB_RECOVERY_FILE_DEST initialization parameters. Oracle ASM then automatically creates control files in the appropriate places.

1.2.2 Multiplex Control Files on Different Disks

The behavior of multiplexed control files is this:

• The database writes to all file names listed for the initialization parameter CONTROL_FILES in the database initialization parameter file.

• The database reads only the first file listed in the CONTROL_FILES parameter during database operation.

• If any of the control files become unavailable during database operation, the instance becomes inoperable and should be terminated.

1.2.3 Back Up Control Files

It is very important that you back up your control files. This is true initially, and every time you change the physical structure of your database.

Such structural changes include:

• Adding, dropping, or renaming data files

• Adding or dropping a tablespace, or altering the read/write state of the tablespace

• Adding or dropping redo log files or groups

1.2.4 Manage the Size of Control Files

The main determinants of the size of a control file are the values set for the MAXDATAFILES, MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, and MAXINSTANCES parameters in the CREATE DATABASE statement that created the associated database.

Increasing the values of these parameters increases the size of a control file of the associated database.

1.3 Creating Control Files

1.3.1 Creating Initial Control Files

The initial control files of an Oracle Database are created when you issue the CREATE DATABASE statement.

CONTROL_FILES = (/u01/oracle/prod/control01.ctl,
                 /u02/oracle/prod/control02.ctl, 
                 /u03/oracle/prod/control03.ctl)

1.3.2 Creating Additional Copies, Renaming, and Relocating Control Files

To add a multiplexed copy of the current control file or to rename a control file:

1. Shut down the database.
2. Copy an existing control file to a new location, using operating system commands.
3. Edit the CONTROL_FILES parameter in the database initialization parameter file to add the new control file name, or to change the existing control file name.
4. Restart the database.

1.3.3 Creating New Control Files

1.3.3.1 When to Create New Control Files

You must create new control files in certain situations.

It is necessary for you to create new control files in the following situations:

• All control files for the database have been permanently damaged and you do not have a control file backup.

• You want to change the database name.

1.3.3.2 The CREATE CONTROLFILE Statement

You can create a new control file for a database using the CREATE CONTROLFILE statement.

The following statement creates a new control file for the prod database (a database that formerly used a different database name):

CREATE CONTROLFILE
   SET DATABASE prod
   LOGFILE GROUP 1 ('/u01/oracle/prod/redo01_01.log', 
                    '/u01/oracle/prod/redo01_02.log'),
           GROUP 2 ('/u01/oracle/prod/redo02_01.log', 
                    '/u01/oracle/prod/redo02_02.log'),
           GROUP 3 ('/u01/oracle/prod/redo03_01.log', 
                    '/u01/oracle/prod/redo03_02.log') 
   RESETLOGS
   DATAFILE '/u01/oracle/prod/system01.dbf' SIZE 3M,
            '/u01/oracle/prod/rbs01.dbs' SIZE 5M,
            '/u01/oracle/prod/users01.dbs' SIZE 5M,
            '/u01/oracle/prod/temp01.dbs' SIZE 5M
   MAXLOGFILES 50
   MAXLOGMEMBERS 3
   MAXLOGHISTORY 400
   MAXDATAFILES 200
   MAXINSTANCES 6
   ARCHIVELOG;

1.3.3.3 Creating New Control Files

1.Make a list of all data files and redo log files of the database.

[oracle@oracle-db-19cs ~]$ 
[oracle@oracle-db-19cs ~]$ sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Thu Mar 2 20:21:47 2023
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0

SQL> show user;
USER is "SYS"
SQL> show con_name;

CON_NAME
------------------------------
CDB$ROOT
SQL> set pagesize 200 linesize 200
SQL> 
SQL> select member from v$logfile;

MEMBER
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u02/oradata/CDB1/redo03.log
/u02/oradata/CDB1/redo02.log
/u02/oradata/CDB1/redo01.log

SQL> select name from v$datafile;

NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u02/oradata/CDB1/system01.dbf
/u02/oradata/CDB1/sysaux01.dbf
/u02/oradata/CDB1/undotbs01.dbf
/u02/oradata/CDB1/pdbseed/system01.dbf
/u02/oradata/CDB1/pdbseed/sysaux01.dbf
/u02/oradata/CDB1/users01.dbf
/u02/oradata/CDB1/pdbseed/undotbs01.dbf
/u02/oradata/CDB1/pdb1/system01.dbf
/u02/oradata/CDB1/pdb1/sysaux01.dbf
/u02/oradata/CDB1/pdb1/undotbs01.dbf
/u02/oradata/CDB1/pdb1/users01.dbf
/u02/oradata/CDB1/pdb2/system01.dbf
/u02/oradata/CDB1/pdb2/sysaux01.dbf
/u02/oradata/CDB1/pdb2/undotbs01.dbf
/u02/oradata/CDB1/pdb2/users01.dbf
/u02/oradata/CDB1/pdb1/data01.dbf
/u02/oradata/CDB1/pdb1/TBS_MAXWELL_01.dbf
/u02/oradata/CDB1/pdb1/undotbs02.dbf
/u02/oradata/CDB1/F4C9A1F8230F2E5CE0538A08A8C02747/datafile/o1_mf_system_kyv81s6t_.dbf
/u02/oradata/CDB1/pdb1/TBS_TOOLS01.dbf
/u02/oradata/CDB1/F4C9A1F8230F2E5CE0538A08A8C02747/datafile/o1_mf_sysaux_kyv81s6v_.dbf
/u02/oradata/CDB1/F4C9A1F8230F2E5CE0538A08A8C02747/datafile/o1_mf_undotbs1_kyv81s6w_.dbf
/u02/oradata/CDB1/F4C9A1F8230F2E5CE0538A08A8C02747/datafile/o1_mf_users_kyv82cng_.dbf
/u02/oradata/CDB1/pdb1/rman01.dbf

24 rows selected.

SQL> select value from v$parameter where name = 'control_files';

VALUE
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u02/oradata/CDB1/control01.ctl, /u02/oradata/CDB1/control02.ctl

SQL> 

2.Shut down the database.

If the database is open, shut down the database normally if possible. Use the IMMEDIATE or ABORT clauses only as a last resort.

3.Back up all data files and redo log files of the database.

4.Start up a new instance, but do not mount or open the database.

STARTUP NOMOUNT

5.Create a new control file for the database using the CREATE CONTROLFILE statement

6.Store a backup of the new control file on an offline storage device.

7.Edit the CONTROL_FILES initialization parameter for the database to indicate all of the control files now part of your database as created in step 5 (not including the backup control file). If you are renaming the database, edit the DB_NAME parameter in your instance parameter file to specify the new name.

8.Recover the database if necessary. If you are not recovering the database, skip to step 9.

If the new control file was created using the RESETLOGS clause, you must specify USING BACKUP CONTROL FILE. If you have lost online redo logs, archived redo log files, or data files, use the procedures for recovering those files.

9.Open the database using one of the following methods:

If you did not perform recovery, or you performed complete, closed database recovery in step 8, open the database normally.

ALTER DATABASE OPEN;

If you specified RESETLOGS when creating the control file, use the ALTER DATABASE statement, indicating RESETLOGS.

ALTER DATABASE OPEN RESETLOGS

1.5 Backing Up Control Files

Use the ALTER DATABASE BACKUP CONTROLFILE statement to back up your control files.

• Back up the control file to a binary file (duplicate of existing control file) using the following statement:

ALTER DATABASE BACKUP CONTROLFILE TO '/oracle/backup/control.bkp';

• Produce SQL statements that can later be used to re-create your control file:

ALTER DATABASE BACKUP CONTROLFILE TO TRACE;

1.6 Recovering a Control File Using a Current Copy

You can recover your control file from a current backup or from a multiplexed copy.

1.6.1 Recovering from Control File Corruption Using a Control File Copy

If a control file becomes corrupted, then you can recover it using a control file copy.

This method assumes that one of the control files specified in the CONTROL_FILES parameter is corrupted, that the control file directory is still accessible, and that you have a multiplexed copy of the control file.

1.With the instance shut down, use an operating system command to overwrite the bad control file with a good copy:

% cp /u03/oracle/prod/control03.ctl  /u02/oracle/prod/control02.ctl

2.Start SQL*Plus and open the database:

SQL> STARTUP

1.6.2 Recovering from Permanent Media Failure Using a Control File Copy

If there is permanent media failure, then you can recover by using a control file copy.

This method assumes that one of the control files specified in the CONTROL_FILES parameter is inaccessible due to a permanent media failure and that you have a multiplexed copy of the control file.

1.With the instance shut down, use an operating system command to copy the current copy of the control file to a new, accessible location:

% cp /u01/oracle/prod/control01.ctl  /u04/oracle/prod/control03.ctl

2.Edit the CONTROL_FILES parameter in the initialization parameter file to replace the bad location with the new location:

CONTROL_FILES = (/u01/oracle/prod/control01.ctl,
                 /u02/oracle/prod/control02.ctl, 
                 /u04/oracle/prod/control03.ctl)

3.Start SQL*Plus and open the database:

SQL> STARTUP

If you have multiplexed control files, you can get the database started up quickly by editing the CONTROL_FILES initialization parameter. Remove the bad control file from CONTROL_FILES setting and you can restart the database immediately. Then you can perform the reconstruction of the bad control file and at some later time shut down and restart the database after editing the CONTROL_FILES initialization parameter to include the recovered control file.

1.7 Dropping Control Files

You can drop control files, but the database should have at least two control files at all times.

You want to drop control files from the database, for example, if the location of a control file is no longer appropriate.

1. Shut down the database.
2. Edit the CONTROL_FILES parameter in the database initialization parameter file to delete the old control file name.
3. Restart the database.

1.8 Control Files Data Dictionary Views

ou can query a set of data dictionary views for information about control files.

The following views display information about control files:

View	Description
V$DATABASE	Displays database information from the control file
V$CONTROLFILE	Lists the names of control files
V$CONTROLFILE_RECORD_SECTION	Displays information about control file record sections
V$PARAMETER	Displays the names of control files as specified in the CONTROL_FILES initialization parameter

SQL> SELECT NAME FROM V$CONTROLFILE;

NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u02/oradata/CDB1/control01.ctl
/u02/oradata/CDB1/control02.ctl

SQL>