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Question and Answer

Tom Kyte

Thanks for the question, Shakeel.

Asked: December 26, 2005 - 11:45 pm UTC

Last updated: October 16, 2006 - 5:41 pm UTC

Version: 9.2

Viewed 10K+ times! This question is

You Asked

hi tom

what is differece between DATA GUARD and REAL APPLICATION CLUSTER.


and Tom said...

everything - absolutely everything.

Besides the fact that they are both part of High Availability - they are night and day different.

RAC provides for high availability (and scalability) "in a room". You have two or more computers sharing a common set of disk. All of the computers have read write access to the data in the single (one) database. Users are logged into the database using every computer that you have hooked together into this cluster. If one computer (node) fails in the cluster - work continues as before - the database never goes down. You have less CPU's perhaps during this time, but the database continues running. This protects from system failures, hardware failures, and many software failures (instance failure, OS panic, whatever).

Data Guard however provides for continuity of operations - if the room in which your RAC cluster resides "goes away" (fire, flood, explosion, whatever) data guard is "somewhere else, ready to take over". Data Guard is primarily about failureover in the event of a catastrophic loss of the primary site.

Data Guard provides other features as well - such as the ability to run in a delayed apply mode - so if you "accidently" truncate a table in production (human error) - you can get the data back from your data guard database, instead of backups. You can offload your backups to the data guard site, you can use the data guard site for readonly reporting if you want. But primarily - data guard is about having a fail over site sitting there read to go in the event of something really bad happening to the primary site.


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What is the difference between Data Guard and Standy Database.

A reader, December 27, 2005 - 2:06 pm UTC

Hi Tom,

What is the difference between Data-Guard and Standy-Database. If there is one!


Tom Kyte
December 27, 2005 - 2:27 pm UTC

consider them synonymous for all intents and purposes - data guard is an extra layer that automates the failover database stuff (shipping of logs, log application, GUI, etc ...)

you could do a standby in 8i and before - but it wasn't data guard, data guard add bits on top of having a simple standby that automate it for us.

Does Standby-database still exists?

A reader, December 27, 2005 - 3:04 pm UTC


Thanks for the quick feedback. Does the Standby-database still exists, as a product, in 9i and 10g -- or it has been phased out already.


Tom Kyte
December 27, 2005 - 3:09 pm UTC

It isn't really a "product" in as much "a feature"

</code> <code>

ctl-f for You Probably Don't Need DataGuard - running a standby database on Standard Edition

Licensing Issue

A reader, December 30, 2005 - 11:40 pm UTC

Thank you for quick responce.
Is There Any Licensing Issue Using DATA GUARD AND RAC


Tom Kyte
December 31, 2005 - 10:58 am UTC

no "issues", issue means "problem"

You have to license RAC with enterprise edition (comes with standard edition).
You have to have licenses for your data guard site

</code> <code>


Parag J Patankar, August 29, 2006 - 7:30 am UTC


I am in process of setting up physical standby database on 9.2. I want to log shipping from LGWR, and I want database protection mode should be maximum availability.

So in log_archive_dest_2 of primary database I have set following parameters 

SQL>show parameters log_archive_dest_2

log_archive_dest_2 string SERVICE=stndby LGWR SYNC AFFIRM

So I belive whatever I do changes should "shipped" to standby site by redo data. ( I have created standb redo logs on standby database )

But on primary database if I do changes in tables and commit, These changes are not getting applied to standby database after I open it as read only. In alerts ( production + standby ) it does not show me anything wrong.

On standby site it shows me

log_archive_dest_1 it shows me ora-00354 corrupt redo block header

My questions regarding this

Q1/ How do I correct this error ? 
Q2/ If I setup log shipping thru LGWR SYNC AFFIRM how logs will get applied to standby ? If I do changes in table on primary ( very small change ) and if I open standby database can I see changes immediately ? ( assume that there is no network issue ) am I correct ?

thanks & regards

Great differentiation between products

A reader, September 27, 2006 - 1:13 pm UTC

Great differentiation between products - was a blur to me the difference, then again I only heard about them as intangibles that our business does not seem to work with. But getting the clear distinction and detailed explanation helps it getting my company to want to start using these tools.


What is RAC cluster guard

A reader, October 16, 2006 - 1:33 pm UTC

In 9i there was a RAC cluster guard which is not present in 10g. Is this merged into Data Guard?

Tom Kyte
October 16, 2006 - 5:41 pm UTC

That was a holdover utility from 8i parallel server days as an optimization for fast failover from node 1 to node 2 (in effect - we used one node, the other was "warm" for a fast failover.

with RAC - became obsolete, with 10g, obscured from view ;)


A reader, September 30, 2012 - 5:16 pm UTC

awesome simple explanation. i notice the people that really know what they are talking about explain things in a simple way.