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Limitations of Flashback Database(from metalink)

原创 Linux操作系统 作者:cqubityj 时间:2008-01-03 13:34:32 0 删除 编辑
The Flashback Database allows you to flash the entire database back to a specific point-intime.
It is best used as a replacement for incomplete recovery of a complete database. The main benefit of the Oracle Flashback Database over incomplete database recovery is that the Flashback Database is much quicker and more efficient. The Flashback Database is not based on undo data but on flashback logs.

If flashback database is enabled, its flashback logs are stored in the Flash Recovery area.

Flashback logs are written sequentially During normal database operation, and they are not archived. Oracle automatically creates, deletes, and resizes Flashback logs in the flash recovery area. You only need to be aware of Flashback logs for monitoring performance and deciding how much disk space to allocate to the flash recovery area for Flashback logs.

The amount of time it takes to Flashback a database is proportional to how far back you need to revert the database, rather than the time it would take to restore and recover the whole database, which could be much longer. The before images in the Flashback logs are only used to restore the database to a point in the past, and forward recovery is used to bring the database to a consistent state at some time in the past. Oracle returns datafiles to the previous point-in-time, but not auxiliary files, such as initialization parameter files.

DB_FLASHBACK_RETENTION_TARGET A parameter value that determines how far back in time you can recover the flashback database, This value is in minutes.

The setting of the DB_FLASHBACK_RETENTION_TARGET initialization parameter determines, indirectly, how much flashback log data the database retains. The size of flashback logs generated by the database for a given time period can vary considerably, however, depending on the specific database workload. If more blocks are affected by database updates during a given interval, then more disk space is used by the flashback log data generated for that interval.

The V$FLASHBACK_DATABASE_LOG view can help you estimate how much space to add to your flash recovery area for flashback logs. After you have enabled logging for Flashback Database and set a flashback retention target, allow the database to run under a normal workload for a while, to generate a representative sample of flashback logs. Then run the following query:

SQL> SELECT ESTIMATED_FLASHBACK_SIZE FROM V$FLASHBACK_DATABASE_LOG;
Limitations of Flashback Database :

Because Flashback Database works by undoing changes to the datafiles that exist at the moment that you run the command, it has the following limitations:

  • Flashback Database can only undo changes to a datafile made by an Oracle database. It cannot be used to repair media failures, or to recover from accidential deletion of datafiles.
  • You cannot use Flashback Database to undo a shrink datafile operation.
  • If the database control file is restored from backup or re-created, all accumulated flashback log information is discarded. You cannot use FLASHBACK DATABASE to return to a point in time before the restore or re-creation of a control file.
  • When using Flashback Database with a target time at which a NOLOGGING operation was in progress, block corruption is likely in the database objects and datafiles affected by the NOLOGGING operation. For example, if you perform. a direct-path INSERT operation in NOLOGGING mode, and that operation runs from 9:00 to 9:15 on April 3, 2005, and you later need to use Flashback Database to return to the target time 09:07 on that date, the objects and datafiles updated by the direct-path INSERT may be left with block corruption after the Flashback Database operation completes.
If possible, avoid using Flashback Database with a target time or SCN that coincides with a NOLOGGING operation. Also, perform. a full or incremental backup of the affected datafiles immediately after any NOLOGGING operation to ensure recoverability to points in time after the operation. If you expect to use Flashback Database to return to a point in time during an operation such as a direct-path INSERT, consider performing the operation in LOGGING mode.

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