June 22, 2005 at 9:40 am
This is a design question and I know largely depends on the use but I would like to get some input from other DBAs.
Our client has asked us to look at scaling down a 3 + 1 3 active/1 passive node Windows 2003 cluster to a 2 node active/passive Windows 2003 cluster. Further to that, our approach to use an instance of SQL Server on each of the 3 active nodes is now potentially going to be come one instance of SQL Server on the active/passive 2 node configuration with all application databases on the one instance.
I'm looking for pros and cons of both of these approaches from a DBA perspective.
Thanks,
Jessica
June 27, 2005 at 8:00 am
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June 28, 2005 at 11:21 am
The benefit of using the n+1 cluster where n>1 is that it allows you to more effectively use a greater percentage of your hardware resources. Effectively a 1+1 Active/Passive setup is a 50% loss of hardware. A 2+1 setup is a 33% loss etc...
The benefit of fewer nodes in the cluster is primarily one of software costs. If a single server can handle the current and anticipated load then there is no reason to license two servers or three servers. Since you are talking about Enterprise Edition and probably a per processor licensing model each processor is around $20k, so hardware costs quickly become less of a concern than software.
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If most people are not willing to see the difficulty, this is mainly because, consciously or unconsciously, they assume that it will be they who will settle these questions for the others, and because they are convinced of their own capacity to do this. -Friedrich August von Hayek
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June 28, 2005 at 11:31 am
Thanks for the thoughts. Could you please elaborate a little more on the following comment. I'm looking specifically for comments around database impacts/benefits.
The benefit of using the n+1 cluster where n>1 is that it allows you to more effectively use a greater percentage of your hardware resources. Effectively a 1+1 Active/Passive setup is a 50% loss of hardware. A 2+1 setup is a 33% loss etc...
Thanks!!!
June 28, 2005 at 12:12 pm
Well, if you have a 1+1 "Active/Passive" cluster setup you have one server that is actually doing work and one that is doing nothing but waiting for the other one to fail. Assuming identical hardware for each (the most common setup) you have lost 50% of your hardware resources. The benefit of n+1 clustering is that you can have multiple servers that are actually doing work and a single inactive failover node, thus increasing your hardware utilization percentage.
In terms of the actual effects on a given database, as long as whatever server the database is hosted on has enough horsepower to adequately handle the load, there should be no difference at all. This is because the specified database (actually the SQL Server instance) runs on only one of the servers regardless of how many servers you have in your cluster.
I hope this addresses your question.
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If most people are not willing to see the difficulty, this is mainly because, consciously or unconsciously, they assume that it will be they who will settle these questions for the others, and because they are convinced of their own capacity to do this. -Friedrich August von Hayek
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