RE: Active/Passive "high availability"

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-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Jared Still Sent: Friday, April 13, 2007 1:42 PM
To: Allen, Brandon
Cc: oracle-l_at_freelists.org
Subject: Re: Active/Passive "high availability"  

On 4/13/07, Allen, Brandon <Brandon.Allen_at_oneneck.com> 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
http://www.oracle.com/technology/products/database/clustering/pdf/ds_rac .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 SAN = SPOF
1 Database = SPOF

I like this diagram for an HA cluster:
http://en.wikipedia.org/wiki/High-availability_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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