6
Managing Control Files

This chapter explains how to create and maintain the control files for your database and contains the following topics:

• What Is a Control File?
• Guidelines for Control Files
• Creating Control Files
• Troubleshooting After Creating Control Files
• Backing Up Control Files
• Recovering a Control File Using a Current Copy
• Dropping Control Files
• Displaying Control File Information

  See Also: Chapter 3, "Using Oracle-Managed Files" for information about creating control files that are both created and managed by the Oracle database server

What Is a Control File?

Every Oracle database has a control file. A control file is a small binary file that records the physical structure of the database and includes:

• The database name
• Names and locations of associated datafiles and online redo log files
• The timestamp of the database creation
• The current log sequence number
• Checkpoint information

The control file must be available for writing by the Oracle database server whenever the database is open. Without the control file, the database cannot be mounted and recovery is difficult.

The control file of an Oracle database is created at the same time as the database. By default, at least one copy of the control file is created during database creation. On some operating systems the default is to create multiple copies. You should create two or more copies of the control file during database creation. You might also need to create control files later, if you lose control files or want to change particular settings in the control files.

Guidelines for Control Files

This section describes guidelines you can use to manage the control files for a database, and contains the following topics:

• Provide Filenames for the Control Files
• Multiplex Control Files on Different Disks
• Place Control Files Appropriately
• Back Up Control Files
• Manage the Size of Control Files

Provide Filenames for the Control Files

You specify control file names using the CONTROL_FILES initialization parameter in the database's initialization parameter file (see "Creating Initial Control Files"). The instance startup procedure recognizes and opens all the listed files. The instance writes to and maintains all listed control files during database operation.

If you do not specify files for CONTROL_FILES before database creation, and you are not using the Oracle Managed Files feature, Oracle creates a control file and uses a default filename. The default name is operating system specific.

Multiplex Control Files on Different Disks

Every Oracle database should have at least two control files, each stored on a different disk. If a control file is damaged due to a disk failure, the associated instance must be shut down. Once the disk drive is repaired, the damaged control file can be restored using the intact copy of the control file from the other disk and the instance can be restarted. In this case, no media recovery is required.

The following describes the behavior of multiplexed control files:

• Oracle writes to all filenames listed for the initialization parameter CONTROL_FILES in the database's initialization parameter file.
• The first file listed in the CONTROL_FILES parameter is the only file read by the Oracle database server during database operation.
• If any of the control files become unavailable during database operation, the instance becomes inoperable and should be aborted.

  Note: Oracle strongly recommends that your database has a minimum of two control files and that they are located on separate disks.

Place Control Files Appropriately

As already suggested, each copy of a control file should be stored on a different disk drive. One practice is to store a control file copy on every disk drive that stores members of online redo log groups, if the online redo log is multiplexed. By storing control files in these locations, you minimize the risk that all control files and all groups of the online redo log will be lost in a single disk failure.

Back Up Control Files

It is very important that you back up your control files. This is true initially, and at any time after you change the physical structure of your database. Such structural changes include:

• Adding, dropping, or renaming datafiles
• Adding or dropping a tablespace, or altering the read-write state of the tablespace
• Adding or dropping redo log files or groups

The methods for backing up control files are discussed in "Backing Up Control Files".

Manage the Size of Control Files

The main determinants of a control file's size 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.

Creating Control Files

This section describes ways to create control files, and contains the following topics:

• Creating Initial Control Files
• Creating Additional Copies, Renaming, and Relocating Control Files
• Creating New Control Files

Creating Initial Control Files

The initial control files of an Oracle database are created when you issue the CREATE DATABASE statement. The names of the control files are specified by the CONTROL_FILES parameter in the initialization parameter file used during database creation. The filenames specified in CONTROL_FILES should be fully specified and are operating system specific. The following is an example of a CONTROL_FILES initialization parameter:

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

If files with the specified names currently exist at the time of database creation, you must specify the CONTROLFILE REUSE clause in the CREATE DATABASE statement, or else an error occurs. Also, if the size of the old control file differs from the SIZE parameter of the new one, you cannot use the REUSE option.

The size of the control file changes between some releases of Oracle, as well as when the number of files specified in the control file changes. Configuration parameters such as MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES, and MAXINSTANCES affect control file size.

You can subsequently change the value of the CONTROL_FILES initialization parameter to add more control files or to change the names or locations of existing control files.

Creating Additional Copies, Renaming, and Relocating Control Files

You can create an an additional control file copy by copying an existing control file to a new location and adding the file's name to the list of control files. Similarly, you rename an existing control file by copying the file to its new name or location, and changing the file's name in the control file list. In both cases, to guarantee that control files do not change during the procedure, shut down the instance before copying the control file.

To Multiplex or Move Additional Copies of the Current Control Files

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

Creating New Control Files

This section discusses when and how to create new control files.

When to Create New Control Files

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 one of the permanent database parameter settings originally specified in the CREATE DATABASE statement. These settings include the database's name and the following parameters: MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES, and MAXINSTANCES.

  For example, you would change a database's name if it conflicted with another database's name in a distributed environment. Or, as another example, you can change the value of MAXLOGFILES if the original setting is too low.

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 (formerly a database that 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') 
   NORESETLOGS
   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;

Steps for Creating New Control Files

Complete the following steps to create a new control file.

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

   If you follow recommendations for control file backups as discussed in "Backing Up Control Files" , you will already have a list of datafiles and online redo log files that reflect the current structure of the database. However, if you have no such list, executing the following statements will produce one.

   SELECT MEMBER FROM V$LOGFILE;
   SELECT NAME FROM V$DATAFILE; 
   SELECT VALUE FROM V$PARAMETER WHERE NAME = 'CONTROL_FILES';
   
   

   If you have no such lists and your control file has been damaged so that the database cannot be opened, try to locate all of the datafiles and online redo log files that constitute the database. Any files not specified in Step 5 are not recoverable once a new control file has been created. Moreover, if you omit any of the files that make up the SYSTEM tablespace, you might not be able to recover the database.

2. Shut down the database.

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

3. Back up all datafiles and online 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.

   When creating a new control file, select the RESETLOGS option if you have lost any online redo log groups in addition to control files. In this case, you will need to recover from the loss of the redo logs (Step 8). You must also specify the RESETLOGS option if you have renamed the database. Otherwise, select the NORESETLOGS option.

6. Store a backup of the new control file on an offline storage device. See "Backing Up Control Files" for instructions for creating a backup.
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 to specify the new name.
8. Recover the database if necessary. If you are not recovering the database, skip to Step 9.

   If you are creating the control file as part of recovery, recover the database. If the new control file was created using the NORESETLOGS option (Step 5), you can recover the database with complete, closed database recovery.

   If the new control file was created using the RESETLOGS option, you must specify USING BACKUP CONTROL FILE. If you have lost online or archived redo logs or datafiles, 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;
     
     

The database is now open and available for use.

Troubleshooting After Creating Control Files

After issuing the CREATE CONTROLFILE statement, you may encounter some common errors. This section describes the most common control file usage errors, and contains the following topics:

• Checking for Missing or Extra Files
• Handling Errors During CREATE CONTROLFILE

Checking for Missing or Extra Files

After creating a new control file and using it to open the database, check the alert file to see if Oracle has detected inconsistencies between the data dictionary and the control file, such as a datafile that the data dictionary includes but the control file does not list.

If a datafile exists in the data dictionary but not in the new control file, Oracle creates a placeholder entry in the control file under the name MISSINGnnnn (where nnnn is the file number in decimal). MISSINGnnnn is flagged in the control file as being offline and requiring media recovery.

The actual datafile corresponding to MISSINGnnnn can be made accessible by renaming MISSINGnnnn so that it points to the datafile only if the datafile was read-only or offline normal. If, on the other hand, MISSINGnnnn corresponds to a datafile that was not read-only or offline normal, then the rename operation cannot be used to make the datafile accessible, because the datafile requires media recovery that is precluded by the results of RESETLOGS. In this case, you must drop the tablespace containing the datafile.

In contrast, if a datafile indicated in the control file is not present in the data dictionary, Oracle removes references to it from the new control file. In both cases, Oracle includes an explanatory message in the alert.log file to let you know what was found.

Handling Errors During CREATE CONTROLFILE

If Oracle sends you an error (usually error ORA-01173, ORA-01176, ORA-01177, ORA-01215, or ORA-01216) when you attempt to mount and open the database after creating a new control file, the most likely cause is that you omitted a file from the CREATE CONTROLFILE statement or included one that should not have been listed. In this case, you should restore the files you backed up in Step 3 and repeat the procedure from Step 4, using the correct filenames.

Backing Up Control Files

Use the ALTER DATABASE BACKUP CONTROLFILE statement to back up your control files. You have two options:

1. 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';
   
   

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

   ALTER DATABASE BACKUP CONTROLFILE TO TRACE;
   
   

   This command writes a SQL script to the database's trace file where it can be captured and edited to reproduce the control file.

   See Also: For more information on backing up your control files, depending upon your backup stratagy refer to one of the following books: Oracle9i User-Managed Backup and Recovery Guide Oracle9i Recovery Manager User's Guide

Recovering a Control File Using a Current Copy

This section presents ways that you can recover your control file from a current backup or from a multiplexed copy.

Recovering from Control File Corruption Using a Control File Copy

This procedure assumes that one of the control files specified in the CONTROL_FILES parameter is corrupted, the control file directory is still accessible, and 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 /u01/oracle/prod/control03.ctl  /u01/oracle/prod/control02.ctl
   
   

2. Start SQL*Plus and open the database:

   SQL> STARTUP
   

Recovering from Permanent Media Failure Using a Control File Copy

This procedure assumes that one of the control files specified in the CONTROL_FILES parameter is inaccessible due to a permanent media failure, and 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
   
   

In any case where you have multiplexed control files, and you must get the database up in minimum time, you can do so by editing the CONTROL_FILES initialization parameter to remove the bad control file and restarting 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.

Dropping Control Files

You can drop control files from the database. For example, you might want to do so if the location of a control file is no longer appropriate. Remember that the database must have at least two control files at all times.

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

   Note: This operation does not physically delete the unwanted control file from the disk. Use operating system commands to delete the unnecessary file after you have dropped the control file from the database.

Displaying Control File Information

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	Can be used to display the names of control files as specified in the CONTROL_FILES initialization parameter

This example lists the names of the control files.

SQL> SELECT NAME FROM V$CONTROLFILE;

NAME
-------------------------------------
/u01/oracle/prod/control01.ctl
/u02/oracle/prod/control02.ctl 
/u03/oracle/prod/control03.ctl