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Looking into PUE calculation for a small datacenter, I have here some questions :

  • With standard air-cooled servers, it is simple, get the power draw of the servers (i.e. IT power), get the power draw of the total facility (i.e. facility power). Divide the latter by the former, you get your PUE.
  • With servers cooled by a building-level system with a cold water delivery circuit and heat exchangers. I am a bit confused whether the water cooler is considered as part of the IT equipment or part of the facility equipment.

If I ask it is because mechanically transporting the heat using water can lead to significant gain in overall efficiency but, if I understand it right, it can negatively affect the PUE if the external part is not counted as IT equipment. This seems to me that there is then a flaw in the metric. I would like here to see here whether it is indeed flawed or whether my understanding is failing.

Let me illustrate with an example, I do on one single server and simple architecture just for the sake of simplicity, but the general idea remains the same :

Server 1 :

  • 200W of power consumption, excluding the fans (i.e. M/B + CPU + RAM + storage | in other words, the actual computing power)
  • 30W of fans in the server.
  • 50W of incurred extra facility power (lighting, efficiency of the power delivery, building ventilation...)

I get here a PUE of 280/230 = 1.22

Server 2 :

  • 200W of power consumption, excluding the fans, (i.e. same hardware as Server 1)
  • 10W for the water circulating pump, located in the server
  • 20W of fans, located on the external water cooling unit, here just a simple radiator with low speed fans (as we are able to work with bigger slower fan instead of the high speed in the air-cooled server) .
  • 50W of incurred extra facility power (lighting, efficiency of the power delivery, building ventilation,...)

I get here a PUE of 280/210 = 1.33

In both I have same hardware and same overall facility power, yet, the PUE is quite different.

So, should I take PUE ratings with a pinch of salt when considering datacenter?

Extra question : is there a metric that calculate the Total power / Computing power? or something like that?

Memes
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2 Answers2

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So, it appears that the PUE only defines

PUE = total Facility Power / IT Power

But it fails to accurately defines where does the IT power enveloppe starts, some include the Rack level power conditionning into the IT Power, some don't. For the cooling, when it comes to Rack level, it is unclear whether included or not, when it is in the server itself, it looks like included, when it is out of the datacenter room, then it is usually to facility power.

Well, it appears that the PUE is as inaccurate as I thought unfortunately.

The metric I was after is, so far only a proposal with the tPUE suggested on this blog entry. the tPUE however is so complete that it may become difficult to implement.

Memes
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PUE is about efficiency - usually that's done for ecological reasons, so you'd need a guesstimate for how much power the building system uses to cool your datacenter.

Any power put into cooling outside the IT equipment - by you or by building management - is considered overhead and counts on the facility side.

Zac67
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  • Thanks for the response, it does not however answer my question about how any Rack level cooling is taken into account. I have got some more answers from James Hammilton blog though https://perspectives.mvdirona.com/2009/06/pue-and-total-power-usage-efficiency-tpue/ – Memes Nov 24 '22 at 08:30
  • Sorry, I hadn't registered that discrepancy - the usual approach is to look at cooling *inside* and *outside* your IT hardware. Standard internal fans are counted as IT equipment as well even though they're required for cooling. Whether you count pumps external to but dedicated to specific hardware as IT or facility isn't really defined anywhere, I'm afraid. – Zac67 Nov 24 '22 at 08:41
  • yes, this is also what I found, making the PUE not totally accurate as we can game the result by including or not some parts of the cooling system in IT equipment.. – Memes Nov 25 '22 at 10:06
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    To have fairly accurate figures I'd count dedicated pumps as IT equipment but heat exchanger as facility. Obviously, PUE numbers created by different people don't really compare... – Zac67 Nov 25 '22 at 10:12