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 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.

DC power storage arrays are also more expensive than AC versions. "You're paying the premium for the more expensive power supply, but it's not dramatic," says EMC's Krone. McClure says Dot Hill's 5730 array, which can scale to 108TB of capacity, lists for $32,000. The price for the DC version would be approximately $500 more per power supply, with two power supplies in each 5730.

But with electrical power beginning to be priced on cycles, "if you went to DC power and had a DC backup system that provided four hours of backup, you could go two to three hours on your batteries and not have to buy peak price electricity," notes McClure.

There are other DC variations on the market. Validus DC Systems, a six-year-old company in Brookfield, CT, offers a hybrid AC-DC power system it claims can cut energy consumption by as much as 40% by eliminating AC power inside the data center, and running server and storage supplies at a higher DC voltage.

Validus CEO Rudy Kraus believes AC power has no place in the data center, just as it has never had a place in telcos. AC power allows electricity to be transmitted over long distances and at high voltages in relatively inexpensive wires, says Kraus. And considering that all the equipment in a data center runs at 12 volts DC, it doesn't make much sense to bring AC inside the data center, step it down a bunch of times, and then convert it into DC power inside the power supply for a server or storage array, he says.

However, "long legacy standards are hard to displace, so I think the uptake on DC power will be slow and deliberate," says Dave Reinsel, group VP, storage and semiconductor research at IDC (and co-author of the worldwide power and cooling study noted earlier). "Other power conservation strategies will likely be adopted first, such as increasing data center temperatures, data center HVAC design and tweaks, storage efficiency like dedupe and optimized disk drives."

--Peter Bochner

This was first published in September 2008

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