Nonetheless, says Bermender, "it was pretty much outdated as soon as it was built." Each rack of blade servers consumes between 18kw and 19kw when running at full tilt. The room's design specification called for six racks per row, but ILM is currently able to fill only two cabinets in each because it literally ran out of outlets. The two power-distribution rails under the raised floor are designed to support four plugs per cabinet, but the newer blade-server racks require between five and seven. To fully load the racks, Bermender had to borrow capacity from adjacent cabinets.

The other limiting factor is cooling. At both ILM and Trinity, the equipment with the highest power density is the blade servers. Trinity uses 8-foot-tall racks. "They're like furnaces. They produce 120-degree heat at the very top," Roberts says. Such racks can easily top 20kw today, and densities could exceed 30kw in the next few years.

What's more, for every watt of power used by IT equipment in data centers today, another watt or more is typically expended to remove waste heat. A 20kw rack requires more than 40kw of power, says Brian Donabedian, an environmental consultant at Hewlett-Packard Co. In systems with dual power supplies, additional power capacity must be provisioned, boosting the power budget even higher. But power-distribution problems are much easier to fix than cooling issues, Donabedian says, and at power densities above 100 watts per square foot, the solutions aren't intuitive.

For example, a common mistake data center managers make is to place exhaust fans above the racks. But unless the ceiling is very high, those fans can make the racks run hotter by interfering with the operation of the room's air conditioning system. "Having all of those produces an air curtain from the top of the rack to the ceiling that stops the horizontal airflow back to the AC units," Roberts says.

Trinity addressed the problem by using targeted cooling. "We put in return air ducts for every system, and we can point them to a specific hot aisle in our data center," he says.