CIO —

Choosing just enough virtual machines, but not too many, for a given server has always been a challenge. Running a set of virtual servers and the applications that they support on one physical server running just one operating system seems easy enough — at first.

But making sure the hardware can support that additional load is a real trick because of the almost infinite variety of the software that runs within the virtual environment — each application making a slightly different set of demands on the host OS and the hardware, says Chris Wolf, analyst at The Burton Group.

Consolidating physical servers into VMs should save money of course, but you can't scrimp too much on the hardware without dragging down the performance of the applications — and risk aggravating end users, says Ian Scanlon, IS operations manager for Computacenter (CCC), a data center and IT services company based in London but covering most of Europe.

"If you put five VMs on a server, you're running six operating systems and all the applications, so you have to ramp up to be able to handle that and keep the service levels, the performance, high for the applications," Scanlon said. "We ended up having to put on a lot more memory than we figured during the capacity planning."

[ For timely virtualization news and expert advice on strategy, see CIO.com's Virtualization Drilldown section. ]

Getting detailed and accurate estimations of how well a server will perform as a VM host is complicated further by the varying ability of different chipsets to support virtual workloads and hypervisors, according to Gordon Haff, high-performance computing analyst at Illuminata.

Virtual machines stress a processor's cache memory harder than a physical server does, and processors differ in their ability to switch between the demands of applications and hypervisors, he says.

Both Intel (INTC) and AMD build in circuits specifically to support both virtualization and the migration of virtual servers. A given server could have between two and eight processors, each of which has between two and eight processing cores. How well your particular server configuration will fare with an idiosyncratic load of software is almost impossible to predict without very specific and painstaking analysis, says Andi Mann, analyst at Enterprise Management Associates (EMA).

Even asking vendor technical or sales reps directly won't get you a specific answer, without your looking at the workloads you intend to put on the server. While there aren't any hard-and-fast rules, a couple of rules of thumb can get you close enough that you'll be able to spot the weak points and where or how to reinforce them, says Massimo Re Ferre, a senior IT architect in IBM's (IBM) Systems and Technology Group. First, for every core on a new Intel or AMD processor you can add three to five virtual machines, he says.

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