U.S.Navy servicemen and -women don their blues, whites or khakis, depending on the occasion and task at hand. For systems engineers at the Navy's Surface Combat Systems Center, however, the color of choice these days is green.

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"With storage, the entire world has long dealt with performance and availability. Now we're adding in a green component," says Les Martin, a civilian tactical systems engineer at the SCSC, in Wallops Island, Va. For the SCSC, that means automatically powering down -- but not turning off -- drives when they're not in use. This giant "sleeping SAN," as he calls it, potentially could reduce the SCSC's annual power costs by 40%, he says.

For this storage-area network (SAN), which houses 168TB of data, SCSC uses the Xiotech Magnitude 3D 3000 Series Fibre Channel system. Energy-efficient 750GB Serial Advanced Technology Attachment (SATA) drives, made by Seagate Technology, fill the chassis.

The SAN's drives each consume 12 watts per hour during normal read/write mode. This compares to the 13W per hour required by the smaller 500GB SATA drives typically used by industry at the time of the Navy's procurement, Martin says.

"That might not seem like much at first read, but we are able to achieve a 7.75% automatic energy reduction in normal read/write consumption while achieving a 50% increase in storage space in a seamless and highly virtualized system," he adds.

For reasons of national security, Martin says, the SCSC must not completely power down the idle SAN drives as some enterprise IT organizations might do. A 750GB drive consumes 0.8W of power when in sleep state, but can be powered up again in about four seconds. That's roughly equivalent to the amount of time a screen image takes to materialize once a cell phone is flipped open, he notes.