Sunday, April 01, 2007

Does Your Virualization Strategy Have a Blind Spot?

I just finished reading a very interesting article about virtualization. It described the test results of running two sample web applications under virtual and physical environments. The idea was simply to check how the virtualization affected the application's performance. The results are interesting.

Before I go further, let's clarify that this article is about Windows IIS and Win2k3 server, so it is not about Solaris, or any other flavor UNIX beyond the fact that the underlying OS for VMWare in this case was CentOS. None the less I believe the story's moral is highly applicable to any operating environment, definitely including UNIX.

There is a lot of detail in the article, but if we can assume their methodology to be relevant, we can reduce it down to a short extract from its conclusion: "a virtualized server running a typical web application may experience a 43% loss of total capacity when compared to a native server running on equivalent hardware."


Of course, I have no reason to think that this function carries over directly to Zones, Xen, or even VMWare running Linux. To believe this without research would be to abandon one's Jedi training. It would be a very valuable experiment to try such an excercise though, because I have no reason (at this moment) to think it would not have some relevance.

The big point here I want to make is that I would bet there are a great many sites out there who dilligently track low hanging fruit metrics like CPU utilization, and use that metric both in planning and asessing their virtualization projects. Server "A" runs an application and on average sits at 10% utilization. Lets call it 100Mhz and find it a home on the consolidation farm. Not so fast.

The problem is that if you completely ignore business metrics, you will fail to identify this article's identified flaw in virtualization. Hey, we moved from 8 servers down to one, and its at 80% all the time. Success! But what if that new server is running at 80% and only processing 50% of the volume it used to? I would suggest that the article I referenced might be a great reason to reflect on your site's metric stratgey and see if you have the ability to accurately asess a consolidaton stratgy. If not, you could be making an expensive transition.

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