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RAC with multiples databases

Hi all,

Can you tell me if multiple databases on one 2 nodes RAC is supported ?
Can you explain me the inconvenient to have multiple databases on RAC ?

Thanks.

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Permalink Reply by Elizabeth Warren on May 15, 2013 at 5:58pm

hey Mike,

An Oracle database is a physical database container. An Oracle schema is a logical database (and comparable to a mySQL or SQL-Server database). A schema can be physically separated from other schemas (different tablespaces used), can have different access rights and privs than other schemas, and even be configured for different resource utilisation than other schemas.

So usually when talking "databases" in general RDBMS conversation, you are referring to an Oracle schema - and not a physical database container which is called "the database" in Oracle speak.

The problem with using multiple database instances on the same server, is duplicating the resource footprint of the database instance each time around. Each db instance needs memory. Needs a number of system processes. Needs CPU cycles. Needs file system space.

Valid reasons are needed for duplicating the resource footprint of multiple database instances on the same server. It seldom makes technical sense from a server manageability, performance and scalability perspective.

So no - it usually does not make sense to have multiple database instances on the same server platform. Oracle does not scale by adding more database instances to a server. Oracle scales by adding more servers and instances to a single database - and this is what RAC is.

Yes, you can have a RAC cluster with multiple databases, using policy based RAC. This entails creating server pools, and assigning servers and RAC databases to these server pools.

best of luck

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Permalink Reply by Matthew Perry on May 15, 2013 at 6:10pm

in reply to Elizabeth:

There are always EXCEPTIONS. So running multiple RAC database instances on a single server is a possibility... just make sure you know EXACTLY what the impact is to RAC wide database settings (some instances for a RAC db may have all the local server resources available, while others need to share resources), what the impact to scalability is, and that this approach is required to meaningfully address the requirements at hand. (though I would be very hard pressed to agree and support such an approach on any of the RACs I look after).

cheers