31 Performing User-Managed Recovery: Advanced Scenarios

If the disk and file system containing the lost control file are intact, then you can simply copy an intact control file to the location of the missing control file. In this case, you do not have to edit the CONTROL_FILES initialization parameter.

To replace a damaged control file by copying a multiplexed control file:

If the disk and file system containing the lost control file are not intact, then you cannot copy a good control file to the location of the missing control file. In this case, you must alter the CONTROL_FILES initialization parameter to indicate a new location for the missing control file.

To restore a control file to a nondefault location:

If possible, restore the control file to its original location. In this way, you avoid having to specify new control file locations in the initialization parameter file.

To restore a backup control file to its default location:

If you cannot restore the control file to its original place because the media damage is too severe, then you must specify new control file locations in the server parameter file. A valid control file must be available in all locations specified by the CONTROL_FILES initialization parameter. If not, then the database prevents you from the mounting the database.

To restore a control file to a nondefault location:

Follow the steps in "Recovering with a Backup Control File in the Default Location ", except after Step 2 add the following step:

Edit all locations specified in the CONTROL_FILES initialization parameter to reflect the new control file locations. Assume that the control file locations listed in the server parameter file are as follows, and both disks are inaccessible:

CONTROL_FILES='/disk1/oradata/trgt/control01.dbf',
              '/disk2/oradata/trgt/control02.dbf'

You can edit the initialization parameter file and specify accessible locations, as shown in the following example:

CONTROL_FILES='/disk3/cf/control01.dbf','/disk4/cf/control02.dbf'

If database recovery with a backup control file rolls forward through a CREATE TABLESPACE or an ALTER TABLESPACE ADD DATAFILE operation, then the database stops recovery when applying the redo record for the added files and lets you confirm the file names.

For example, suppose that the following sequence of events occurs:

1. You back up the database.

2. You create a tablespace containing the following data files: /disk1/oradata/trgt/test01.dbf and /disk1/oradata/trgt/test02.dbf.

3. You restore a backup control file and perform media recovery through the CREATE TABLESPACE operation.

You may see the following error when applying the CREATE TABLESPACE redo data:

ORA-00283: recovery session canceled due to errors 
ORA-01244: unnamed datafile(s) added to control file by media recovery
ORA-01110: data file 11: '/disk1/oradata/trgt/test02.dbf'
ORA-01110: data file 10: '/disk1/oradata/trgt/test01.dbf'

To recover through an ADD DATAFILE operation:

If you have a read-only tablespace on read-only or slow media, then you may encounter errors or poor performance when recovering with the USING BACKUP CONTROLFILE option. This situation occurs when the backup control file indicates that a tablespace was read/write when the control file was backed up. In this case, media recovery may attempt to write to the files. For read-only media, the database issues an error saying that it cannot write to the files. For slow media, such as a hierarchical storage system backed up by tapes, performance may suffer.

To avoid these problems, use current control files rather than backups to recover the database. If you must use a backup control file, then you can also avoid this problem if the read-only tablespace has not suffered a media failure. You have the following alternatives for recovering read-only and slow media when using a backup control file:

• Take data files from read-only tablespaces offline before doing recovery with a backup control file, and then bring the files online after media recovery.

• Use the correct version of the control file for the recovery. If the tablespace is read-only when recovery completes, then the control file backup must be from a time when the tablespace was read-only. Similarly, if the tablespace is read/write after recovery, then the control file must be from a time when the tablespace was read/write.

You can recover backups through an OPEN RESETLOGS operation so long as:

• You have a current, backup, or created control file that detects prior incarnations

• You have all available archived redo logs

If you must re-create the control file, then the trace file generated by ALTER DATABASE BACKUP CONTROLFILE TO TRACE contains the necessary commands to reconstruct the complete incarnation history. The V$DATABASE_INCARNATION view displays the RESETLOGS history of the control file, and the V$LOG_HISTORY view displays the archived log history.

It is possible for the incarnation history to be incomplete in the in re-created control file. For example, archived logs necessary for recovery may be missing. In this case, it is possible to create incarnation records explicitly with the ALTER DATABASE REGISTER LOGFILE statement.

In the following example, you register four logs that are necessary for recovery but are not recorded in the re-created control file, and then recover the database:

ALTER DATABASE REGISTER LOGFILE '/disk1/oradata/trgt/arch/arcr_1_1_42343523.arc';
ALTER DATABASE REGISTER LOGFILE '/disk1/oradata/trgt/arch/arcr_1_1_34546466.arc';
ALTER DATABASE REGISTER LOGFILE '/disk1/oradata/trgt/arch/arcr_1_1_23435466.arc';
ALTER DATABASE REGISTER LOGFILE '/disk1/oradata/trgt/arch/arcr_1_1_12343533.arc';
RECOVER AUTOMATIC DATABASE;

If a current or backup control file is unavailable for recovery, then you can execute a CREATE CONTROLFILE statement. Do not list read-only files in the CREATE CONTROLFILE statement so recovery can skip them. No recovery is required for read-only data files unless you restored backups of these files when the data files were read/write.

After you create a control file and attempt to mount and open the database, the database performs a data dictionary check against the files listed in the control file. For each file that is not listed in the CREATE CONTROLFILE statement but is present in the data dictionary, an entry is created for them in the control file. These files are named as MISSINGnnnnn, where nnnnn is a 5-digit number starting with 0.

After the database is open, rename the read-only files to their correct file names by executing the ALTER DATABASE RENAME FILE statement for all the files whose names are prefixed with MISSING.

To prepare for a scenario in which you might have to re-create the control file:

Run the following statement when the database is mounted or open to obtain the CREATE CONTROLFILE syntax:

ALTER DATABASE BACKUP CONTROLFILE TO TRACE;

The preceding SQL statement produces a trace file that you can edit and use as a script to re-create the control file. You can specify either the RESETLOGS or NORESETLOGS (default) keywords to generate CREATE CONTROLFILE ... RESETLOGS or CREATE CONTROLFILE ... NORESETLOGS versions of the script.

All the restrictions related to read-only files in CREATE CONTROLFILE statements also apply to offline normal tablespaces, except that you must to bring the tablespace online after the database is open. Omit temp files from the CREATE CONTROLFILE statement and add them after opening the database.

You can recover after losing a member of a multiplexed online redo log group. The database continues to function as usual during the following conditions:

If the online redo log of a database is multiplexed, and if at least one member of each online redo log group is not affected by the media failure, then the database continues functioning as usual, but error messages are written to the log writer trace file and the alert_SID.log of the database.

You can resolve the problem of a missing member of a multiplexed online redo log group by taking one of the following actions:

• If the hardware problem is temporary, then correct it. The log writer process accesses the previously unavailable online redo log files as if the problem never existed.

• If the hardware problem is permanent, then drop the damaged member and add a new member by using the following procedure.

  Note:

  The newly added member provides no redundancy until the log group is reused.

If a media failure damages all members of an online redo log group, then different scenarios can occur depending on the type of online redo log group affected by the failure and the archiving mode of the database.

If the damaged online redo log group is current and active, then it is needed for crash recovery; otherwise, it is not. Table 31-4 outlines the various recovery scenarios.

To determine whether the damaged group is active or inactive.