Oracle9i Backup and Recovery Concepts
Release 1 (9.0.1)

Part Number A90133-02
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Backup and Recovery Strategies

This chapter offers guidelines and considerations for developing an effective backup and recovery strategy. It includes the following topics:

Backup Strategies

Before you create an Oracle database, decide how to protect the database against potential media failures. If you do not develop a backup strategy before creating your database, then you may not be able to perform recovery if a disk failure damages the datafiles, online redo log files, or control files.

This section describes general guidelines that can help you decide when to perform database backups and which parts of a database you should back up. Of course, the specifics of your strategy depend on the constraints under which you are operating.

This section contains these topics:

Obeying the Golden Rule of Backup and Recovery

The set of files needed to recover from the failure of any Oracle database file--a datafile, control file, or online redo log--is called the redundancy set. The redundancy set contains:

The golden rule of backup and recovery is: the set of disks or other media that contain the redundancy set should be separate from the disks that contain the datafiles, online redo logs, and control files. This strategy ensures that the failure of a disk that contains a datafile does not also cause the loss of the backups or redo logs needed to recover the datafile. Consequently, a minimal production-level database requires at least two disk drives: one to hold the files in the redundancy set and one to hold the database files.

Always keep the redundancy set separate from the primary files in every way possible: on separate volumes, separate file systems, and separate RAID devices. These systems are reliable, but they can and do fail. Keeping the redundancy set separate ensures that you can recover from a failure without losing committed transactions.

You can implement a system that follows the golden rule in several different ways. Oracle recommends following these guidelines:

Choosing the Database Archiving Mode

Before you create an Oracle database, decide how you plan to protect it against potential failures. Answer the following questions:

After you have answered these questions and determined which mode to use, follow the guidelines for either:

Backing Up a NOARCHIVELOG Database

If you run the database in NOARCHIVELOG mode, Oracle does not archive filled groups of online redo log files. Therefore, the only protection against a disk failure is the most recent whole backup of the database. Follow these guidelines:

Backing Up an ARCHIVELOG Database

If you run your database in ARCHIVELOG mode, then the archiver archives groups of online redo log files. Therefore, the archived redo log coupled with the online redo log and datafile backups can protect the database from a disk failure, providing for complete recovery from a disk failure to the instant that the failure occurred (or, to the desired noncurrent time). Following are common backup strategies for a database operating in ARCHIVELOG mode:

Multiplexing Control Files, Online Redo Logs, and Archived Redo Logs

Control files, online redo logs, and archived redo logs are crucial files for backup and recovery operations. The loss of any of these files can cause you to lose data irrevocably. You should maintain:

Performing Backups Frequently and Regularly

Frequent backups are essential for any recovery scheme. Base the frequency of backups on the rate or frequency of database changes such as:

If users generate a significant amount of DML, then database backup frequency should be proportionally high. Alternatively, if a database is mainly read-only, and if updates are issued only infrequently, then you can back up the database less frequently.

You can use either RMAN or user-managed methods to create backup scripts. If you set persistent configurations using RMAN's CONFIGURE command, however, then you should not typically need to write extensive scripts. You can regularly run BACKUP DATABASE PLUS ARCHIVELOG.

See Also:

Oracle9i Recovery Manager User's Guide and Reference to learn how to create, delete, replace, and print stored scripts 

Performing Backups Before and After You Make Structural Changes

Administrators as well as users make changes to a database. If you make any of the following structural changes, then perform a backup of the appropriate portion of your database immediately before and after completing the following changes:

The part of the database that you should back up depends on your archiving mode:

Mode  Action 


Make a control file backup (using RMAN or using the ALTER DATABASE statement with the BACKUP CONTROLFILE option) before and after a structural alteration. Of course, you can back up other parts of the database as well. 


Make a consistent whole database backup immediately before and after the modification. 

Backing Up Often-Used Tablespaces

Many DBAs find that regular whole database backups are not in themselves sufficient for a robust backup strategy. If you run in ARCHIVELOG mode, then you can back up the datafiles of an individual tablespace or even a single datafile. This option is useful if a portion of a database is used more extensively than others, for example, the SYSTEM tablespace and automatic undo tablespaces.

By making more frequent backups of the extensively used datafiles of a database, you avoid a long recovery time. For example, you may make a whole database backup once every two weeks. If the database experiences heavy traffic during the week, then a media failure on Friday can force you to apply a tremendous amount of redo during recovery. If you had backed up your most frequently accessed tablespaces three times a week, then you could apply a smaller number of changes to roll the restored file forward to the time of the failure.

If you are running in automatic undo management mode, then be sure to regularly back up your undo tablespaces. If you run in manual undo management mode, then be sure to regularly back up all tablespaces containing rollback segments.

See Also:

Oracle9i Database Administrator's Guide for information about managing undo tablespaces 

Performing Backups After Unrecoverable Operations

If users are creating tables or indexes using the UNRECOVERABLE option, then make backups after the objects are created. When tables and indexes are created as UNRECOVERABLE, Oracle does not log redo data, which means that you cannot recover these objects from existing backups.


If using RMAN, then you can make an incremental backup. 

See Also:

Oracle9i SQL Reference for information about the UNRECOVERABLE option of the CREATE TABLE ... AS SELECT and CREATE INDEX statements. 

Performing Whole Database Backups After Opening with the RESETLOGS Option

After you open a database with the RESETLOGS option, Oracle Corporation recommends that you immediately perform a whole database backup. If you do not, and if a disaster occurs, then it is possible to lose all changes made after opening the database.

In certain cases, you can restore a backup made prior to a RESETLOGS and recover the database, but the procedure is complicated and requires you to have a control file backup from before and after the RESETLOGS operations. A whole database backup created after a RESETLOGS protects against this situation.

See Also:

Oracle9i Recovery Manager User's Guide and Reference to learn how to recover using a backup created before a RESETLOGS 

Archiving Older Backups

You may need to store older backups for two basic reasons:

If you want to recover to a noncurrent time, then you need a database backup that completed before the desired time. For example, if you make backups on the 1st and 14th of February, then decide at the end of the month to recover your database to February 7th, you must use the February 1st (or earlier) backup.

For a database operating in NOARCHIVELOG mode, the backup that you use must be a consistent whole database backup. Of course, you cannot perform media recovery using this backup. For a database operating in ARCHIVELOG mode, the whole database backup:

For added protection, keep two or more database backups (with associated archived redo logs) previous to the current backup. Thus, if your most recent backups are not usable, then you will not lose all of your data.

Knowing the Constraints for Distributed Database Backups

If the database is a member of a distributed database system, then all databases in the system should operate in the same archiving mode. Note the consequences and constraints contained in the following table.

Mode  Constraint  Consequence 


Closed cleanly 

Backups at each node can be performed autonomously, that is, individually and without time coordination. 


Closed cleanly 

Consistent whole database backups must be performed at the same global time to plan for global distributed database recovery. For example, if a database in New York is backed up at midnight EST, the database in San Francisco should be backed up at 9 PM PST. 

See Also:

Oracle9i Database Administrator's Guide to learn how to manage distributed database systems 

Exporting Data for Added Protection and Flexibility

Because the Oracle Export utility can selectively export specific objects, consider exporting portions or all of a database for supplemental protection and flexibility in a database's backup strategy. This strategy is especially useful for logical backups of the RMAN recovery catalog, because you can quickly reimport this data into any database and rebuild the catalog if the recovery catalog database is lost.

Database exports are not a substitute for whole database backups and cannot provide the same complete recovery advantages that the built-in functionality of Oracle offers. For example, you cannot apply archived logs to logical backups in order to update lost changes. An export provides a snapshot of the logical data (tables, stored procedures, and so forth) in a database when the export was made.

See Also:

Oracle9i Database Utilities for an account of the Export utility 

Avoiding the Backup of Online Redo Logs

Although it may seem that you should back up online redo logs along with the datafiles and control file, this technique is dangerous. You should not back up online redo logs for the following reasons:

A number of situations are possible in which restoring the online logs cause significant problems to the database. The following sections describe scenarios that illustrate how restoring backed up online logs severely compromises recovery.

Unintentionally Restoring Online Redo Logs: Scenario

When a crisis occurs, it is easy to make a simple mistake. When restoring the whole database, you can accidentally restore the online redo logs, thus overwriting the current online logs with the older, useless backups. This action forces you to perform incomplete recovery instead of the intended complete recovery, thereby losing the ability to recover valuable data contained in the overwritten redo logs.

Erroneously Creating Multiple Parallel Redo Log Timelines: Scenario

If you face a problem where the best course of action is to restore the database from a consistent backup and not perform any recovery, then you may think it is safe to restore the online logs and thereby avoid opening the database with the RESETLOGS option. The problem is that Oracle eventually generates a log sequence number that was already generated by the database during the previous timeline.

For example, say that the most recent archived log for database prod1 has a log sequence number of 100. Assume that you restore a consistent backup of the database along with backed up online redo logs and then do not open with the RESETLOGS option. Assume also that the restored online log is at log sequence 50. Eventually, the database archives a log with the log sequence number of 100--so you now have two copies of log 100 with completely different contents.

If you then face another disaster and need to restore from this backup and roll forward, then you may find it difficult to identify which log with sequence number 100 is the correct one. If you had reset the logs, then you would have created a new incarnation of the database. You could only apply archived logs created by this new incarnation to this incarnation.


RMAN does not permit you to back up online redo logs. 

Keeping Records of the Hardware and Software Configuration of the Server

During the stress of a recovery situation, it is important that you have all necessary information at your disposal. This is especially true if for some reason you need to contact Oracle Support because you run into a problem that you do not understand. You should have the following documentation about the hardware configuration:

You should also keep the following documentation about the software configuration:

You should keep this information both in electronic and hardcopy form. For example, if you save this information in a text file on the network or in an email message, then if the entire system goes down, you may not have this data available.

Restore and Recovery Strategies

Oracle provides a variety of procedures and tools to assist you with recovery. To develop an effective recovery strategy, do the following:

Testing Backup and Recovery Strategies

Practice backup and recovery techniques in a test environment before and after you move to a production system. In this way, you can measure the thoroughness of your strategies and minimize problems before they occur in a real situation. Performing test recoveries regularly ensures that your archiving, backup, and recovery procedures work. It also helps you stay familiar with recovery procedures, so that you are less likely to make a mistake in a crisis.

If you use RMAN, then run the DUPLICATE command to create a test database using backups of your production database. If you perform user-managed backup and recovery, then you can either create a new database, a standby database, or a copy of an existing database by using a combination of operating system and SQL*Plus commands.

When testing your backup and recovery strategy, ask yourself these questions:

Validating Backups and Restores Using RMAN

If you use RMAN, then you can use the VALIDATE keyword on the BACKUP and RESTORE commands. BACKUP VALIDATE tests whether you are able to make a valid backup of database files. RESTORE VALIDATE tests whether you are able to restore an RMAN backup. Note that neither of these commands produces any actual output files.

Planning a Response to Media Failures

Media failure is the biggest threat to your data. A media failure is a physical problem that occurs when a computer unsuccessfully attempts to read from or write to a file necessary to operate the database. Common types of media problems include:

The technique you use to recover from media failure of a database file depends heavily on the type of media failure that occurred. For example, the strategy you use to recover from a corrupted datafile is different from the strategy for recovering from the loss of the control file.

The basic steps for media recovery are:

When you perform datafile media recovery, you choose either complete recovery or incomplete recovery. The type of recovery method you use depends on the situation. Table 4-1 displays typical scenarios and strategies.

Table 4-1 Typical Media Failures and Recovery Strategies  
Lost/Inaccessible Files  Archiving Mode  Status  Strategy 

One or more datafiles 



Restore whole database from a consistent database backup. All changes made after the backup are lost. Open the database with the RESETLOGS option.

Note: The only time you can open a database without performing RESETLOGS after restoring a NOARCHIVELOG backup is when you have not already overwritten the online log files that were current at the time of the most recent backup. 

One or more datafiles and an online redo log 



Restore whole database from consistent backup. You lose all changes made after the last backup. Open the database with the RESETLOGS option. 

One or more datafiles and all control files 



Restore the whole database and control file from consistent backup. You lose all changes made after the last backup. Open the database with the RESETLOGS option. 

One or more (but not all) datafiles 



Perform tablespace or datafile recovery while the database is open. The tablespaces or datafiles are taken offline, restored from backups, recovered, and placed online. No changes are lost and the database remains available during the recovery.  

All datafiles 



Restore the backup datafiles, then mount the control file and recover the database completely. Assuming all redo logs are available, you can open the database as normal (that is, do not perform a RESETLOGS). 

One or more online redo log groups 



Perform incomplete recovery of the database up to the point of the lost online redo log. Open the database with the RESETLOGS option. 

One or more datafiles and an archived redo log required for recovery 



Perform TSPITR on the tablespaces containing the lost datafiles up to the point of the latest available archived redo log. 

All control files and possibly one or more datafiles 


Not open 

Restore the lost control files and datafiles from backups and recover the datafiles. No changes are lost, but the database is unavailable during recovery. Open the database with the RESETLOGS option. 

All control files and possibly one or more datafiles, as well as an archived or online redo log required for recovery 


Not open 

Restore the necessary files from backups, then perform incomplete recovery of the database up to the point of the most recent available log. You will lose all changes contained in the lost log and in all subsequent logs. Open the database with the RESETLOGS option. 

Planning a Response to Datafile Block Corruption

If selected blocks within a datafile are corrupt, then you may not have to restore and recover the whole datafile. Instead, you can perform block media recovery. The Recovery Manager BLOCKRECOVER command can restore and recover specified data blocks while the database is open and the corrupted datafile is online.

See Also:

Oracle9i Recovery Manager User's Guide and Reference to learn how to perform block media recovery 

Planning the Response to Non-Media Failures

Although media recovery is your primary concern when developing your recovery strategy, you should understand the basic types of non-media failures as well as the causes and solutions for each.

Statement Failure

A statement failure is a logical failure in the handling of a statement in an Oracle program. The Oracle database server or the operating system usually returns an error code and a message when a statement failure occurs.

User Error

Users errors are any mistakes that users make in adding data to or deleting data from the database. If you have a logical backup of a table from which data has been lost, sometimes you can simply import it back into the table.

Depending on the scenario, you may have to perform some type of incomplete media recovery to correct user errors. You can perform either database point-in-time recovery (DBPITR) or tablespace point-in-time recovery (TSPITR). The following table explains the difference between these types of incomplete recovery.

Type  Description 


  1. Restore whole database backup.

  2. Recover the database to the time just before the error.




  1. Create auxiliary instance with RMAN or user-managed methods.

  2. Recover the tablespace on the auxiliary to the time just before the error.

  3. Import data back into the primary database.


Instance Failure

Instance failure occurs when an instance abnormally terminates. An instance failure can occur because:

Fortunately, Oracle performs instance recovery automatically: all you need to do is restart the database. Oracle automatically detects that the database was not shut down cleanly, then applies committed and uncommitted redo records in the redo log to the datafiles and rolls back uncommitted data. Finally, Oracle synchronizes the datafiles and control file and opens the database.

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