April 21, 2006

Keeping Your Cool in the Data Center

by Michael Feldman

As more computational muscle is incorporated into blade servers, clusters and supercomputers, the resulting increases in power and heat have become a significant challenge for the data center. Power-hungry blade servers, in particular, have become a major source of thermal pollution. Additional IT equipment, such as routers and other communication gear, are also contributing to the power and heat loads. The IT manager is left trying to reconcile the increases in computational demand with the ability of the data center to accommodate it.

Solutions are emerging. Technological advances in processors, such as lower-power chips, multi-core processors and on-chip power management are being developed to slow power demands. But in the short-term, computational demand is overwhelming these solutions. Fortunately, companies specializing in power and cooling equipment have developed strategies that address even the most power-demanding computing centers.

One of these companies, American Power Conversion (APC), has a variety of solutions for powering and cooling the modern data center. APC provides these solutions for thousands of data centers around the world for both commercial and non-commercial organizations. Last month, Richard Sawyer, APC’s Data Center Technology director, presented a tutorial session at the High Performance Computing and Communications (HPPC) conference in Newport, Rhode Island, to educate conference attendees about some of the latest power and cooling strategies that the industry has to offer.

Sawyer gives these types of presentations to help educate IT professionals about how the industry has progressed in the past few years in terms of solving the high-heat-density problem. The evolution from mainframes to blades is occurring rapidly and many IT managers are unaware of the types of strategies that have recently become available to solve the ensuing power and heat dilemma.

Blades reared their ugly head about three years ago,” explained Sawyer. “Manufacturers were all of sudden dealing with [power] densities of 5 to 20KW per rack, which led to a lot of hot spots. The hot spots were what drew everybody’s attention; we developed fixes for that. In the past, they always designed data centers around the power reliability array. Today it’s all about cooling.”

According to APC studies, blade servers require about 20 times the power and cooling of the average data center design. In the past five years, blade server power density has increased rapidly to the point where systems of 24KW per rack are becoming common. A 24KW blade rack generates the heat equivalent to 2 electric ranges. This year, IBM has been talking about driving its Blue Gene/L technology – currently at 31KW per rack — into its BladeCenter products.

“In the last two years, we poured a lot of money into solving the high-heat-density problem in the data center,” said Sawyer. “There’s some interesting technology out there, but it forces a little bit of a rethinking on how to design data centers.”

As a first step in determining a facility’s power and cooling requirements, APC will run a 3D computational fluid dynamics (CFD) analysis using the known data center parameters. Once the model is built, they incorporate the intended IT equipment into the virtual data center. For example, if they’re going to add a couple of blade chassis, they simply plug them into the model and perform a what-if scenario. That lets them know pretty quickly where the capacity is going to be used up and what potential problems could occur.

“Then we reach into our bag of technological tricks and come up with a [solution] that solves that particular problem for them,” said Sawyer.

And just what are those technological tricks? According to Sawyer, the whole industry is moving towards the concept of close-coupling, which means putting the cooling units as close as possible to the source of heat. Instead of arranging a data center with rows of racks in the middle with cooling units around the edge of the room, the cooling units are being moved in close proximity to the IT equipment.

Another strategy is to migrate from air cooling to liquid cooling. Liquid is a much better medium for cooling than air. If you have any doubts about this, compare the different effect of sticking your hand in the refrigerator versus plunging it into some cold water. As heat densities increase in the data center, the ability of air cooling to keep temperatures in the optimal range (68F – 77F degrees) becomes problematic.

Sawyer says that when you go over about 140-150 watts per square foot, which equates to about 3 to 4KW per rack, you start to get into trouble. Beyond this power density, you have more cooling equipment than IT equipment in the data center. So the question becomes how to best cool the equipment, but preserve use of that space. To do this, you have to go to some type of high-density cooling solution.

“There are two things that the users — the IT side of house — have to concede,” said Sawyer. “One is that they are going to have cooling units very close to the racks — in fact, probably in the same row as the racks. The second thing is that there’s going to be some kind of fluid cooling involved — water, glycol or a waterless liquid refrigerant.”

According to Sawyer, that’s not as bad as it sounds, because data centers were originally designed around mainframes, which typically were water-cooled. In fact, raised floors were invented to accommodate the water pipes for mainframe cooling when data centers were first built. Those raised floor are going to be necessary to provide liquid to cooling units that are intermixed with the racks.

But there is resistance to liquid-cooled units by the IT folks. The mantra that Sawyer often hears is: “We don’t want water in our data center.” But, according to Sawyer, they already have to deal with water; the standard air-conditioning units have humidifiers to compensate for the dehumidification that takes places during cooling. And most of the older IT folks are already comfortable with the idea of liquids, since they grew up with water-cooled mainframes.

“It’s a bit of a marketing problem, not just for us, but also for our competitors, to [suggest fluid cooling] in a data center, especially after all these years where we’ve had air cooling,” explained Sawyer. “It’s a little bit of a re-education. So my basic line is: if you’ve got hydrophobia, get over it.”

While power and cooling isn’t the most prominent technology in high-tech facilities, typically representing only 10 to 20 percent of the investment of IT hardware, it is one of the most critical. As Richard Sawyer likes to remind people: “When your server fails, you lose the application; if we fail, you lose all your applications.”