How many instances should be there in RAC to get best performance

We have 4 instances RAC on and ASM.

In order to get the best performance I have below questions.

1. In theory how many instances can be added in RAC (11g)?
2. Practically how many instances can give best performance?
 Note: I think after certain number of instances the performance will be flat, means no more performance benefit after a certain level.

Is there any already proved method with statistics to decide the same?
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Victor TarasovProgrammerCommented:
Oracle says that Rac enhances fault tolerance option, rather than performance. It may be faster query performance due to parallel execution, but this may become a bottleneck IO.
So it is necessary to analyze each system and give personalized recommendations.
RAC is not for performance but scalability and fault tolerance.  

With 9i and 10.1 it was between 4 and 128 depending upon the clusterware
with 10.2 it is 100

Scalability meaning that let's say you have a business where you sell something and your business is growing by leaps and bounds.  so what you do is

1.  Get and customize a great application for your current business needs (2 nodes in a RAC).

2.  Then in couple of years your business has shot through the roof and your RAC can't handle the transactional volume.  Since you bought RAC all you need to do is to Buy another machine and add it to the RAC (from 2 nodes to 3)

and thus in another couple of years you can add another machine and so forth.

Performance in RAC is limited to the interconnect speeds due to Global cache residing in all active nodes.
RAC is not for performance or scalability (this is feature of the application and DB structure).

RAC is for Load Balancing over cheap commodity server machines - this means high load can be divided into many instances installed on cheap servers.

Fault tolerance? No! The DB is centralized. No matter how many instances are running in the RAC if the only DB files collection experience diffuculties (corrupted device for instance) there is not easy way to solve the problem.

Yes - the maximum number of the nodes in one cluster could be 100, but this also means Oracle licences.

About the performance question:

You have to study the RAC wait events:

Another good tool is AWR reports of the RAC instances.

In AWR pay attention to the following:



3. CPU usage
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YBSolutionsAuthor Commented:
Dear schwertner,

Are you sure that 'RAC is not for performance or scalability' ?
If so, please think again and explain in details, may be I didn't get it correctly.
My dear friend YBSolution,

last fall I read the Oracle electronic course "RAC 11g administration". I was greatly surprised when Oracle itself explains that scalability could be easily comprimised by bad application DB design. And Oracle give example:
- 4 RAC instances
- centralized DB (RAC uses 1 (one) single database.
- All applications on the 4 nodes use same table to hold data or to register new actions. Due the locks on commonly used tables and rows there is zero scalability, no matter how many nodes you add to the RAC

If course my attempt to explain the Oracle text is miserable. The original text is 15-20 pages long. But this was a great chapter. Oracle explains in details the difference between performance and scalability .... I am sure you can find electronic copy of this Oracle course somewhere in the net or on the OTN.

Next very interesting thing was that disk cylinders that are peripheral are 4-10 times faster then cylynders close to axes of the disk drive. So ASM has option to place the active tablespaces in the peripheral cylynders ... but only if it knows where they are, because logical volumes mask the real physical location of the disk.

Facing the complexity of RAC and ASM i fear that RAC is marketing trick of Oracle. Adding this complexity the RAC is very hard to be administered, there are so much bugs, that Oracle recomends when patching RAC to create ... clonning of the installation. Could you seriosly accept such advice?

RAC was intensivily made in the times of the dotcom financial catastrophe in the begin of the 21 century. The companies haven't money for expensive SMP (Symetric Multiprocessor Computers)  computers. So the idea of Oracle was to downgrade to cheap servers. But the complexity of a RAC is high, the administration - hard task. The worst thing is that the application (if there are many instances of the application or the application uses frequently common resources) has to be specially written to make advantages from RAC.

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YBSolutionsAuthor Commented:
Hi Dear,

May I get the URL for the article you read?
slightwv (䄆 Netminder) Commented:
>>May I get the URL for the article you read?

It is not an article.  From the post: "Oracle electronic course "

I believe you will find it is an Oracle course:
Everybody knows these facts.

For instance look here:

A Competitive Review of Oracle Real Application Clusters +1'd this publicly. Undo

Furthermore, Oracle RAC best practice says that it cannot make a non-scalable application scale,
and suggests using data partitioning to minimize the traffic ...


In google look for "A Competitive Review of Oracle Real Application Clusters"
YBSolutionsAuthor Commented:
I got the below error while trying to access the URL

Your query didn’t match any course descriptions. Please try again.

Anyways, my question is answered.
slightwv (䄆 Netminder) Commented:
I pull up:
Oracle Database 11g: RAC Administration

  Oracle University provides ILT and LVC students a downloadable eKit containing the training materials for this course.

 Oracle University eKit now viewable on the iPad
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