Oracle Database Instance Recovery Tutorial

Is It Possible to Corrupt an Oracle Database?

No, You Cannot – Not If It Is Properly Administered

Why? Because the mechanism of redo and undo is the best yet developed on this planet.

In this tutorial, Oracle Certified Master John Watson will explain redo, undo, commit, instance recovery and demonstrate why your database can never lose one row of committed data.

Learn what actually happens, in memory and on disc, when DML is issued. John will demonstrate that instance recovery is automatic, and unstoppable.

This is critical and essential knowledge for all Oracle Administrators and DBA students.

Presenter: John Watson, Oracle Certified Master

This free tutorial is segmented into fourteen separate lessons:

  1. Introduction to Oracle Certified Master John Watson and SkillBuilders (1:44)
  2. Tutorial Agenda (2:40)
  3. The Database ACID Test (3:09)
  4. Physical and Logical Corruption (1:37)
  5. Demonstration – What Does Oracle do when DML is Executed? (9:47)
  6. Commit and the Log File Sync Wait Event (1:18)
  7. When Why and How Does Oracle Write to Disk (1:58)
  8. What About Checkpoints? (1:16)
  9. Discussion about COMMIT NOWAIT (3:46)
  10. Lecture – What Oracle Database does when DML is Executed (7:22)
  11. The Oracle Instant Crash Recovery Mechanism (6:24)
  12. Principles to Remember (0:52)
  13. Oracle – The Incorruptible Database (1:54) (click on video below)
  14. Next Steps (0:47)

Date: Jun 4, 2013

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Oracle – The Incorruptible Database

Oracle Instance Recovery Tutorial


Module 13 – Oracle – The Incorruptible Database


>> John:  The result, incorruptible database. What I’ve tried to explain is how the undo-redo mechanism is unbreakable and you can have absolute competence in it. Go through the examples such as I’ve done, sketch it out in your notebook, test what’s actually visible in the various physical files the way you saw me doing it, and you’ll demonstrate to yourself that no matter what combination of circumstances you will never end up with a committed transaction that is lost on uncommitted transaction that survives provided your redo log is available. 




What if it isn’t available? That takes us back to that question earlier. If the instance recovery  fails, the reason it fails is that your redo log is damaged. In that circumstance, you can’t open the database. What does that mean? You got to protect your redo log and that is through the multiplexing mechanism that you must enable to follow your company standards. 


These standards are vital. If your company standards say the redo log must consist of two copies of each log file group on two separate devices and both those devices are damaged, you’ve lost your redo log and you lost data. But you as DBA can then point to the standards document and say, “Hey, that’s what was agreed I would do.” 




You must have standards. Does your standard say one copy, two copy, four copies? I’m not going to say how many you should have. But you need documented standards for how the redo log should be protected and that will protect not only the database but it will also protect you.




 The end result of that is that provided you meet that service level agreement, you and your users can have absolute confidence in Oracle.


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