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Tripping On Power: Page 2 of 14

Paul Frountan, vice president of engineering at managed hosting provider Rackspace, sees it slightly differently. "Our maximum rack configurations draw up to 20kW, but to achieve that we tightly control the airflow into and out of the rack. As long as systems are running at normal temperatures, they'll survive for the 30 seconds needed for generators to take over."

There are other data center problems with packing racks full. Besides localized cooling, there's the issue of weight. Present-day raised floors are rated for a uniform loading of 250lbs/sq ft or less. If you're dealing with one 10 years old or older, it may only be rated for 75lbs/sq ft. Certain earthquake-prone communities may require even lower loading. They may also require that racks maintain a lower center of gravity than usual.

The heaviest configuration cited in our table on page 36 works out to a load of 744lbs/sq ft, or almost triple the best rating of modern raised floors. To address this, Rackspace always builds its data centers on grade and uses extra floor bracing for heavy systems, according to Frountan. In high rise buildings, maximum floor point loads must also be considered, along with uniform loading restrictions.

Problematically, raised floors that can handle a ton in each rack are less likely to allow for the required cooling airflow. That's because this can require a sub-floor-to-raised-floor gap of approximately three feet, whereas most raised floors are designed with less than a two-foot gap. This clearance is sometimes reduced even further to increase the weight-bearing capacity, resulting in an even bigger drop in cooling capacity. In most instances, the problem can be addressed by pressurizing overhead air ducts as well as the floor gap. That way, when floor-based airflow isn't sufficient, it can be augmented by ceiling-based ducts.

There's also the issue of power distribution. Data center design in the mainframe era called for power densities as low as 30W/sq ft, though 75W/sq ft is more typical. Using the 75W reference, our fully stocked HP blade rack would require dedicating the power capacity of 413 square feet of floor space--hardly a practical proposition.