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RE: Active/Passive "high availability"

From: Kerber, Andrew W. <>
Date: Fri, 13 Apr 2007 14:34:32 -0500
Message-ID: <>

Well, of course RAC is not automatically an ha solution, it can be part of an ha solution, assuming all your hardware is mirrored to separate devices on separate channels, etc.  

-----Original Message-----
[] On Behalf Of Jared Still Sent: Friday, April 13, 2007 1:42 PM
To: Allen, Brandon
Subject: Re: Active/Passive "high availability"  

On 4/13/07, Allen, Brandon <> wrote:

        I've noticed there seems to be some disagreement about whether or not RAC is really a "High Availability" solution. There is no doubt that Oracle advertises it as such:          

        E.g. from .pdf:

Kind of the 'Truth in Advertising" concept? ;)                     

        On page 13 there is a graphic of RAC with a mirrored disk subsystem, and again the claim of "No Single Point of Failure"

Last time I looked, a mirrored disk resides in some type of disk farm. SAN, NFS, whatever, it is a SPOF.

SAN failures are not exacly unheard of.          

        So, it seems to come down to how you define a "single point". If you look at the "database" as a single point, then yes there is a SPOF, but if you look at it more granularly and consider that the database resides on hardware with mirrored disks and multiple controllers, fibre channels, fans, power supplies, etc. - then there isn't really a SPOF at the database level either because it would really require multiple failures to bring down the database.

No, it doesn't require multiple failures. I am not a storage expert, but having redundant components in a SAN does not make it HA. It makes it more resistant to failiure, but it cannot be relied on to guarantee a high percentage of availability.

I have also seen SAN's fail more than once.

A single database is also not HA.

A system requiring 99.9% availability allows only 8.76 hours of downtime per year. Can you do that with a single db?

1 Database = SPOF

I like this diagram for an HA cluster:

        A disaster such as flood, fire, earthquake, vandalism, etc. could easily bring down the entire disk array at once, but that falls under the category of Disaster Recovery, not HA and I think we all agree that RAC is generally not a DR solution although I believe some are running RAC with the nodes geographically dispersed in an attempt to incorporate DR as well.

It isn't really necessary to have a physical disaster.

A careless tech can bring down your SAN for quite a long time.

Jared Still 
Certifiable Oracle DBA and Part Time Perl Evangelist

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Received on Fri Apr 13 2007 - 14:34:32 CDT

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