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DC power can cut costs, but conversion isn't easy

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storage arrays that use direct current (DC) electrical power instead of standard alternating current (AC) power could be the next big thing, especially with Framingham, MA-based research firm IDC estimating that the amount spent on powering and cooling all of the external storage arrays worldwide exceeded $1 billion in 2007.

Proponents of DC power say it produces 20% to 40% less heat and is more reliable than AC power. "The primary electrical fuel for the telephone network is DC power," says Jay Krone, EMC's senior director of storage platforms. "And that's why when the lights go out, the phone still works."

While most server vendors offer DC power options, only major storage array vendors like EMC, IBM and Sun are much into DC power options for their products. EMC is on its third generation of DC-power Clariions, and recently qualified its Centera product line for DC power. IBM offers a DC power option for its DS3300 storage array, while Sun has its Netra st D130 storage unit. And in June, OEM storage array maker Dot Hill Systems began offering its midrange 5730 Fibre Channel storage arrays with DC power.

While having storage arrays incorporate DC power supplies seems like a logical extension of the green storage trend, most DC power customers are not Earth Day-observant data

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centers but telcos and the military. And when a DC-powered storage array does end up in a data center, it's usually a newly constructed one. "It's the same for data centers as it is for construction companies--no one likes retrofitting," says Scott McClure, midrange arrays product manager at Dot Hill Systems.

Any piece of equipment in a data center or server room (storage arrays, servers, switches, etc.) can use DC power. In a traditional data center, AC power comes from an outside power source and is then converted into DC several times before reaching the servers or storage arrays. But during each conversion, electricity is lost and heat is generated. For this reason, says Krone, "if you're going to go DC, you really have to go DC" and consider an all-DC network. Just make sure the servers "know" they're receiving DC current.

Another option for a data center that wants to run DC power is to install an AC-to-DC conversion device, a solution that could carry several penalties. First, there's the cost of the conversion unit. On top of that, your storage array won't be as efficient as it was.

"Some Clariions were sold that way, with the extra component in there as a rack, and it wasn't as efficient when [the converter] was in the array," says Krone. "DC power equipment works just like AC power management: the software tools work, the remote diagnostics work. But the converter box is another point of failure." In addition, larger cables are needed to transmit DC power.

This was first published in September 2008

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