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I'd like to get some advice on what factors influence the decision of how large (in terms of resources) you would decide to size each node of a virtual machine cluster (VMware, Xen, etc), and in what way. What are the benefits/drawbacks of multiple smaller individual nodes (less cores, RAM, etc) vs. a smaller number of larger individual nodes? Is there a balance to be struck, and if so, in your experience, what do you consider when attempting to find the optimum?

I appreciate that there almost certainly isn't a one size fits all answer to the question, but what factors would you take into consideration?

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This is a one-size-fits-none question as it's entirely dependent on the kind of workload you intend to run on the cluster... – voretaq7♦ Sep 9 '11 at 14:56
Ok, I was happy to take an answer along the lines of "X & Y type workloads would influence the decision in Z manner." That's why I worded the question as 'what factors would influence.' Suppose it was a general purpose cluster. A good example would be a cloud hosting provider for mainstream use. – Newt Sep 9 '11 at 15:18
It'll come down to cost. If you can afford the costs associated with lots of 4-socket servers, then do it. But the better bargain is smaller 2-socket systems. – ewwhite Sep 9 '11 at 15:26
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It is a factor of workload. But these days, licensing dictates a lot of it. Most of the VMWare installations I've performed have been with smaller organizations, so I had to optimize the number of cores per CPU socket. These days, a good 2U server with plenty of RAM expansion (e.g. HP ProLiant DL380 or Dell R710) is a good bet. Those systems are optimized for virtualization. It's the sweet-spot in terms of price/performance. You'd need to provide more details on what you are looking to do in order for us to give a better answer, though.

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