How Many Virtual Machines Can Run On A Single Server?

How many user desktops can run on a virtualized server? From a structural point of view, this question has a lot in common with, "How many angels can dance on the head of a pin?" Nevertheless, I did my best to answer it in the new McGraw-Hill book, Management Strategies for the Cloud Revolution. As it turns out, I was wrong.In trying to convey the looming power of the cloud over end user computing, I attempted to address the question of how many end users might be served a virtual desktop from a remote point if only one server were available. The answer I came up with was based on a four-way server with each CPU equipped with four cores, a rather common configuration these days. I said such a machine based on Nehalem cores could satisfy the needs of 256 ends users at a time, no problem. In effect, a server built with basic PC parts can match what used to be 256 individual PC desktops.

I had misgivings that many people would believe such an answer. I was trying to push the envelope and worried last October that a commodity server like the one I was using in my example would be unlikely to support so many users in a real world setting. Nevertheless, in Chapter 3, "Virtualization Changes Everything," I stated one commodity server could host that number of end users without causing a serious degradation in the desktop experience they were accustomed to.

Now fast forward to May 17, 2010. It's CA World in Las Vegas, and 7,000 people have turned out to learn about the possibilities of using virtualized servers and cloud computing. On the stage was Edouard Bugnion, the Swiss computer scientist who was an original founder of VMware and until 2004, its first chief architect, now fully engaged as a VP in Cisco Systems effort to become a player in the blade server market. He probably had something to do with Cisco deciding to focus its Unified Computing System blades on the attributes of virtualized servers. He may even have suggested how Cisco could use its networking expertise to ease the impending, I/O bottleneck on servers densely stacked with virtual machines.

At the Las Vegas press conference, CA Technologies senior VP of virtualization Roger Pilc described how CA products can natively manage Cisco UCS blades. Then at the end of the press conference, I got my chance to ask Bugnion my version of "How many angels can dance…" question.

Bugnion paused to think. How many virtualized desktops could be supported by a UCS blade? He referred to the VMark benchmark that Cisco conducted to measure blade performance, which doesn't directly address the issue. Then he answered my question.

"UCS can drive the VM density to higher numbers. We know of instances where 330 desktops are supported on a single blade," he said. When it comes to cloud computing, its advocates and thought leaders are often accused of distorting the facts and making exaggerated claims, both of which raise false hopes. I'm beginning to doubt that the advocates go far enough. Which would you rather pay for and administer? A single centralized blade and adjoining network fabric? Or 330 new laptops whose end users want to customize them with recently downloaded software every time you turn your back?

Server CPU cycles are proliferating in multi-core machines, tied to broadband networks connected by the Internet. A set of simple asynchronous Web conventions can get a powerful server halfway around the world to work with a laptop in your backyard, without any specialized knowledge. Virtualization may not change everything, but it is helping set loose forces that greatly extend the distribution of computer power -- in a new model with a drastically reduced price point. That model is "the cloud." Critics be damned.