2.1 Data Recovery Strategy Determines Backup Strategy

To decide on backup strategies, start with your data recovery requirements and your data recovery strategy. Each type of data recovery will require that you take certain types of backup.

Failures can run the gamut from user error, datafile block corruption and media failure to situations like the complete loss of a data center. How quickly you can resume normal operation of your database is a function of what kinds of restore and recovery techniques you include in your planning. Each restore and recovery technique will impose requirements on your backup strategy, including which features of the Oracle database you use to take, store and manage your backups.

When thinking about recovery strategies, ask yourself questions like these:

  • If a disk failed and destroyed some of the database files, such as datafiles or redo logs, how would you recover the lost files? As described in "Planning a Response to Media Failure: Restore and Media Recovery", you should be able to handle the loss of datafiles, control files, and online redo logs.

  • If a logic error in an application or a user error caused the loss of important data from one or several tables or tablespaces, how could you recover that data, and what would happen to database updates since the error? Could you determine the cause of the error, to prevent it from happening again? As described in "Planning Responses to User Error: Point-in-Time Recovery and Flashback Features", techniques available to you include point-in-time recovery of the whole database or one or more tablespaces, importing data from earlier logical exports with one of the data import utilities, and using the Oracle database's flashback features.

  • If the instance alert log indicates that one or more tables contains corrupt blocks, how can you repair the corruption? Does the tablespace have to remain available during the repair? As described in "Planning a Response to Datafile Block Corruption: Block Media Recovery", the RMAN BLOCKRECOVER command can help you in this situation. Also, troubleshoot recovery with the SQL*Plus RECOVER ... TEST command.

  • If the entire data center is destroyed, can you perform disaster recovery? Assume that all you have is an archive tape containing backups. How would you recover the database? How long would that recovery take?

  • If you were not available to recover your database, could someone else recover it in your absence? Are your recovery procedures sufficiently automated and documented?

With these needs in mind, decide how you can take advantage of features related to backup and recovery, and look at how each feature meets some requirement of your backup strategy. For example:

  • Using Recovery Manager simplifies most backup and recovery operations compared to user-managed backup and recovery. It automates management of most backup files, including the deletion of backups and archived redo logs from disk or tape when no longer needed to meet recovery goals. It provides detailed reporting on backup activities, can verify that your available backups can be used to recover your database. Finally, RMAN makes possible many recovery techniques not available if you are using user-managed backup and recovery, such as incremental backups.

  • Flashback Database will help you restore a database to a previous time much faster than media recovery. However, you must decide in advance to keep flashback logs, and keeping flashback logs requires that you configure a flash recovery area.

  • Block media recovery may be better than datafile media recovery if availability is critical. While block media recovery is possible even if you do not base your backup and recovery strategy on RMAN, RMAN-based block media recovery can be performed more quickly and with less effort.

Once you decide which features to use in your recovery strategy, you can plan your backup strategy, answering the following questions, among others:

  • How and where will you store your recovery-related files? Will you use a flash recovery area? Will you use an ASM disk group to provide redundancy? Will you store backups on tape or other offline storage, or only on disk?

  • At what intervals will you take scheduled backups? And what form of physical backups will you take in each situation?

  • What situations require you to take a database backup outside of the regular schedule? Sometimes you must take an unscheduled backup to ensure that you can recover your data, such as after an OPEN RESETLOGS or after changes to your database such as NOLOGGING operations that do not appear in the redo log. You may also have business requirements that require backups for auditing purposes or other reasons not related to database recovery.

  • How can you validate your backups, to ensure that you can recover your database when necessary?

  • How do you manage records of your backups? Do you use RMAN with a recovery catalog?

  • Do you have detailed recovery plans that cover each type of failure? How do your DBAs can execute these plans in a crisis? Can scripts be written to automate execution of these plans in a crisis?

  • Can you apply Oracle database availability technologies, such as Data Guard or Real Application Clusters, to improve availability during a database failure? How does using these availability technologies affect your backup and recovery strategy?

These are of course only a few of the considerations you should take into account. Available resources (hardware, media, staff, budget, and so on) will also be factors in your decision.