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Saturday, 24 November 2012

Instance-Crash Recovery

Crash recovery or instance recovery is the automatic recovery of the database by the oracle server , without any intervention by the DBA. For example, if a power outage brings down your database instance , when the power supply resumes,you only need to restart the database instance.
The oracle server will use the informaton saved in the onlineredo log files to synchronize the datafiles.Instance recovery involves the following two key operations:

  • Rolling forward:During this operation,the oracle server will update all datafiles with the information from the redo log files.The online redo log-files are always written to before the data is recorded in the datafiles. Thus an instance recovery may usually leave the online log files "ahead" of the data-files.
  • Rolling back:During this operation, uncommitted changes that were added to the datafiles the roll-forward operation are rolled back.Oracle does this by using the undo tablespace contents to return uncommitted changes to return uncommitted changes to their original states.At the end of rollback stage only committed data at the end of time of instance failure is retained in the datafiles.
 During rollforward process , the database server must apply all transactions between the last checkpoint and the end of redo log.You use the parameter fast_start_mttr_target to specify the number of seconds you want the crash recovery to take.Oracle will try to recover the instance as close as possible to the time that you specify for the fast_start_mttr_target parameter.Maximum value of this parameter is 3600 seconds (one hr).