Monday, April 28, 2008

Flashback Part #1 Basics

Flashback Database

Oracle Flashback Database, accessible from both RMAN and SQL*Plus , lets you quickly recover the entire database from logical data corruptions or user errors. It is similar to conventional point in time recovery in its effects, allowing you to return a database to its state at a time in the recent past. Flashback Database is, however, much faster than point-in-time recovery, because it does not require restoring datafiles from backup and it requires applying fewer changes from the archived redo logs.

Flashback Database uses its own logging mechanism, creating flashback logs which are stored in the flash recovery area. You can only use Flashback Database if flashback logs are available. Therefore, you must set up your database in advance to create flashback logs if you want to take advantage of Flashback Database feature.

To enable Flashback Database, you set up a flash recovery area, and set a flashback retention target , to specify how far back into the past you want to be able to restore your database with Flashback Database.

From that time on, at regular intervals, the database copies images of each altered block in every datafile into the flashback logs. These block images can later be reused to reconstruct the datafile contents as of any moment at which logs were captured. When a database is restored to its state at some past target time using Flashback Database, each block changed since that time is restored from the copy of the block in the flashback logs most immediately prior to the desired target time. The redo log is then used to re-apply changes since the time that block was copied to the flashback logs.

Flashback Database Window

The range of SCNs for which there is currently enough flashback log data to support the FLASHBACK DATABASE command is called the flashback database window. If space in the flash recovery area is low, then flashback logs may be deleted to free space for files required by the configured retention policy. The result is that the flashback database window can be shorter than the flashback retention target, depending upon the size of the flash recovery area, other backups that must be retained and how much flashback logging data is needed.

Normal Restore Points

Creating a normal restore point assigns the restore point name to a specific point in time or SCN, as a kind of alias you can use with commands that recognize a RESTORE POINT clause as a shorthand for specifying an SCN. Before performing any operation that you may have to reverse, you can create a normal restore point. The name of the restore point and the SCN are recorded in the control file. Then, if you later need to use Flashback Database, Flashback Table, or point-in-time recovery, you can refer to the target time using the name of the restore point instead of a time expression or SCN.

Normal restore points are very lightweight. The control file can maintain a record of thousands of normal restore points with no significant impact upon database performance. Normal restore points eventually age out of the control file if not manually deleted, so they require no ongoing maintenance.

Guaranteed Restore Points

Like normal restore points, guaranteed restore points can be used as aliases for SCNs in recovery operations. However, they also provide specific functionality related to the use of the Flashback Database feature.

Creating a guaranteed restore point at a particular SCN enforces the requirement that you can perform a Flashback Database operation to return your database to its state at that SCN, even if flashback logging is not enabled for your database. If flashback logging is enabled, creating a guaranteed restore point enforces the retention of flashback logs required for Flashback Database back to any point in time after the creation of the earliest guaranteed restore point.

A guaranteed restore point can be used to revert a whole database to a known good state days or weeks ago, as long as there is enough disk space in flash recovery area to store the needed logs.

Logging for Flashback Database and Guaranteed Restore Points

The logging for Flashback Database and guaranteed restore points is based upon capturing images of datafile blocks before changes are applied, so that these images can be used to return the datafiles to their previous state when a FLASHBACK DATABASE command is executed. The chief differences between normal flashback logging and logging for guaranteed restore points are related to when blocks are logged and whether the logs can be
deleted in response to space pressure in the flash recovery area. These differences affect space usage for logs and database performance.

Requirements for Using Guaranteed Restore Points

  1. The COMPATIBLE initialization parameter must be set to 10.2 or greater.
  2. The database must be running in ARCHIVELOG mode. The FLASHBACK DATABASE operation used to return your database to a guaranteed restore point requires the use of archived redo logs from around the time of the restore point.
  3. A flash recovery area must be configured. Guaranteed restore points use a mechanism similar to flashback logging, and as with flashback logging, Oracle must store the required logs in the flash recovery area.
  4. If flashback database is not enabled, then the database must be mounted, not open, when creating the first guaranteed restore point (or if all previously created guaranteed restore points have been dropped).

Related Topics:

  1. Flashback Part #2 Setup and Maintenance
  2. Features of Oracle Flashback
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