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11g FLASHBACK DATABASE

Oracle 11g New Features Tips by Burleson Consulting
June 29, 2008

Oracle 11g New Features Tips

Last but not least is FLASHBACK DATABASE, which was also introduced in 10gR1, with completely different technology.  Flashback database was not enabled by default and could only be enabled if the database was in ARCHIVELOG mode with the flash recovery area defined. The parameters db_recovery_file_dest and db_trecovery_file_dest_size are used to define a flash recover area. In order to enable flashback database, DBAs had to shutdown and startup mount the database and make an entry into the controlfile with:

ALTER DATABASE FLASHBACK ON;

This would then start the recovery writer, rvwr, background process and allocate the flashback buffer in shared memory.  Keep in mind that the size of the flashback buffer is dependent on the size of the redo log buffer. It reaches its full size of 16M if the redo log buffer has a size of at least 8M.

The rvwr creates the copies of buffers of the buffer cache before they are modified into the flashback buffer and flushes them to disk based on a complex algorithm. Once enabled, Oracle copied before images of buffers  which had been modified in the buffer cache to the flashback buffer.  From there, the rvwr would flush them to disk into the flashback logs of the flash recovery area. These copies could then be used to restore blocks directly in the datafiles during a flashback database operation. This was done using sql*plusrman , or the OEM interface which again would call rman.

Via the flashback_retention_target parameter, users were able to specify when flashback logs would age out from the flashback logs automatically.  This being, how far to the database should flashbacked in the past could be specified.   Also, a restore point was introduced as a named point in time to remember a timestamp or SCN that may be used for a flashback.

Since Oracle Database 10gR2, it has been possible to create guaranteed restore points.  This enables the ability to use flashback for exactly one point in time in the past. As opposed to the normal restore points, these restore point are reliable. They do not age out unless the DBA deletes them, and can be used even if FLASHBACK is not enabled for the database. The flashback logs then hold all block copies which need to flash the database back to the particular point in time. This can be used for situations such as application upgrades.  Here is an example situation ideal for the flashback database:

  • A guaranteed restore point is created before upgrade

  • Upgrade completed

  • Results are unsatisfactory, but the whole upgrade can easily be undone you using

  • flashback database


 

This is an excerpt from the new book Oracle 11g New Features: Expert Guide to the Important New Features by John Garmany, Steve Karam, Lutz Hartmann, V. J. Jain, Brian Carr.

You can buy it direct from the publisher for 30% off.

 

 
  
 

 
 
 
 
 

 
 
 
 
 
 
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