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Total Tyranny of low utilization datacenters

Friday Nov 17, 2006

The Total Tyranny of low utilization datacenters

In this blog and other blogs I've commented on, Woodcrest supporters always want to say their servers are better at low utilisation. This is totally the wrong way to go! They first claim typical datacenters are running at low utilisations, example: Xen claims typical datacenters are at 15%. Horrible, HORRIBLE.

So why shouldn't use just add all kinds of techniques to power at lower utilisations, clearly that is the best way to save money? Right? Wrong.

Lets take a simple example of a 400 watt server(@ 100%) that saves 20 watts for each 10% reduction in utilisation. Will show this in a table below and compare equivalent work done compared to 100% so you can see the hyperbolic nature of the curve. Of course I'm only looking at one server so there is some discretisation but when you have a datacenter it will quickly approach these numbers.

%Utilisation 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Watts-at-Util 400 380 360 340 320 300 280 260 240 220 220

watts/work 400 422 450 486 533 600 700 867 1200 2200 inf.

Now that I've got you shocked, let's look at a more typical example. Lets compare 5 servers running at 10% utilisation (that is 220 watts each or 1100 watts for the 5 of them). A single server running at 50% utilisation only uses 300 watts! The 10% case almost require 3.7 times more power! OUCH!

Bottom line: It is far too easy to be fooled to think you are saving money if power-saving features at low utilisation is your answer. By the by, a significant number of Sun's large servers run at over 80% utilisation using Solaris, of course.

Here is an example from 2004 of someone on different products who likely understands this math. As reported in Computerworld:

"Dennis Callahan, CIO at The Guardian Life Insurance Company of America in New York, server utilization has shot up to nearly 50% in the past 18 months, with a goal in coming years of nearly 70%.

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