Power, Pickups and Polar Bears

Let me give you an example. Let’s compare a Ford Focus and a Ford F-150 pickup. If we wanted to be “green”, looking at the brochures for both vehicles, the fuel economy of the Focus would be a no-brainer (35 hwy vs. 20 hwy for the F-150). But, what if we wanted to actually “do” something with our vehicle? Since I was working on the yard today, let’s say we wanted to haul 2 yards of mulch. Our F-150, with 55 cubic feet of hauling capacity can do this in one trip, while the Focus, with slightly under 14 cubic feet of cargo capacity would take four trips. So, even with the 75% better gas milage, the Focus is not automatically the right choice. The point is a simplistic one dimensional measure is not really all that meaningful. This is why a city bus can get 4 mpg and still be central a “green” strategy. The other point is that we have multiple types vehicles for a reason--so if I were a landscaper, I may drive my F-150 during the week and drive my Focus when I go tooling in Napa for the weekend. The goal is to derive the maximum value for the energy consumed, regardless of if its vehicles or data center switches.

The topic of relevant metrics brings us back to something Dave mentions in his blog: Nortel’s claims that their 45xxT switch consumes 56% less power than an “equivalent” Cisco Catalyst 3750G switch (for the sake of argument, I’ll work with the comparisons that Nortel picked). There is some confusion in the Nortel positioning paper, since the switch model and power consumption noted in the text is different than the model and power consumption noted in the table the text refers to, but I digress. The salient point is that these power values are for two switches plugged-in and idling with no connections--the assertion is that if there is such a discrepancy at idle, imagine what happens when we actually forward packets. Before I go on, a note to those of you with data centers: if you have switches in your data center that are plugged in and not doing anything, please unplug them now--it will help you with power/cooling and the polar bears will thank you too. Now, for the rest of us with data center switches that forward packets, it might be useful to see what a switch does under load.

So, earlier this year, at our request, Miercom did an analysis of a number of switches under load to see what kind of power consumption you might actually encounter in your data center. It turns out the Cisco Catalyst 3750G consumes about 140-150W (depending on packet size and choice of copper or optical uplinks) when those uplinks are driven at 100%. This is inline with the 160W maximum draw noted on the data sheet. What does the Nortel 45xxT do under load? That information is not published anywhere I could find, but perhaps we can make an approximation based on a couple of data points we do have. First, the data sheet for the Nortel 4548GT notes a maximum draw of 150W, so unless Nortel is into really over-engineering their power supplies, that is probably a fair indicator of real-world draw at max load. Second, Miercom did test the Nortel 5510 as part of the testing mentioned earlier and they found that switch drew about 125-130W at 100% uplink load against a maximum draw of 135W in the Nortel 5510 data sheet, so there does not seem to be any revolutionary power management technology at work. Third, the notes from Miercom indicate between a nominal 5% load and a 100% load, the power draw grew proportionally regardless of vendor tested. So, I think it is reasonable to deduce that the Nortel 45xxT is in the same neighborhood as the Cisco Catalyst 3750G when used to actually forward packets. Sidebar: Miercom just announced that the Cisco Catalyst 3750-E, 3560-E, 3750, 3560, and 2960 Series Switches are the first products certified in their “Certified Green” certification program.

Later in the same position paper, Nortel asserts their ERS 8610 offers energy savings of 60% over a “6500 equivalent”. There is not a lot of detail to the comparison, although there is a footnote that states “Unless noted, all product comparisons are based on vendor published maximum power ratings.”, so this seems a comparison of power supplies, not actual draw. Again, this becomes a Focus vs F-150 comparison--what exactly are we comparing and in what context--I could dig into this but I think you get the point.

I guess the moral of the story here is, when it comes to energy efficiency, there is, sadly, no one magic metric to measure goodness. Its a design function like anything else in the data center and its a matter of doing research and balancing the design parameters. Our goal is, simply, to help you make an informed decision. To that end, we have the Power Calculator to give you configuration-specific load numbers for our equipment. We then take it a step further with our Data Center Assurance Program tool on our data center design best practices website, which includes power consumption information as part of its guidance. Check them both out (you will need a cisco.com account).