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Am I Cheating on PUE or DCiE?
Posted on: October 14th, 2010 by Bill

In a parallel blog, we’ve been discussing the death of the modern data center. It’s not as dire or alarmist as it sounds – merely the realization that data centers are evolving as both real estate-based solutions outside the enterprise and the use of cloud computing for social media, entertainment and now more traditional corporate computing functions. In many ways, we’re returning to the days where the MEP infrastructure is specifically engineered to the computing systems they support. For you old fogies out there (like me, even at 47), chilled water’s making a comeback as well. We’re not just dumping capacity into a white space anymore.

Recently, I was reviewing an article in ENR stating that a data center achieved a PUE of 1.08. I will say that I found the figure a bit hard to believe. Where we found fault with the statement was that it was:

  • Not backed by environmental data.
  • Did not appear to be normalized over an annualized basis, since the facility has been open less than one year.
  • Not offered out of context to the entire data center population.
  • Does not mention that it uses a significant amount of server fan power to achieve this figure.

It doesn’t mean the figure was misstated, just taken out of context.

When viewing some of the home run PUE’s of the past couple of years, sites located in areas known for beneficial cooling (like the use of very cold outside air, just off the North or Baltic Sea) were leveled to sites in the US with some explanation. We’re not contending the 1.08 was hokey, just that when compared to a site in Phoenix, Dallas or Northern Virginia, clear disclosure is necessary. It also doesn’t state if there was a reduction of the kW’s consumed – just that the ratio is lower. What you should care about in PUE is the reduction in overall power consumption or an increase in MIPS/W or whatever power-to-IT metric you happen to fancy.

I will also say that the current PUE and DCiE standard is pretty generic. The new ANSI/BICSI Data Center Standard 002 speaks specifically to annualized data, where all four seasons must be considered. The PUE and DCiE figures don’t recognize server fan power in the equation. Nor do we expect that to change in the near future.

Here are some areas that we’re finding suppress PUEvalues:

  • Environmental data for the PUE calculation is not taken on either the winter or summer design days. Per the new ANSI/BICSI Data Center Standard 002, PUE must be examined with design day enviromentals.
  • Many users are now using server fans as part of the ventilation chain in a more formalized way. While this has been the case with forced ventilation cabinets for years, users like Facebook, Google and Yahoo will actually design the server fan into the supply and return air chain. What happens is the server fan power is counted with the IT load and not the supporting load. Fair game, but you’ll see a PUE as much as 25 – 30 basis points lower than a system that uses more traditional air handling system where the server fans are not engineered into ventilation chain.

Make sure that PUE and DCiE consider the server fan contribution where used when comparing them to facilities that don’t employ or consider the server fan power as part of the formal HVAC system.

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