Disk spin-down: Power savings with a catch

Disk spin-down and MAID can reduce power consumption, but spin-up latency and time-outs are drawbacks. Read about one firm's experience with disk spin-down.

Published: 14 Jul 2010
Disk spin-down technology, also known as massive array of idle disks ( MAID), presents a clear-cut opportunity to reduce energy consumption, but IT shops need to take care to use it in the right scenarios.

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Copan Systems Inc., which was acquired by SGI earlier this year, introduced MAID-based systems in 2004 that allowed a maximum of 25% of their disks to be powered on at the same time. Lessening the power and cooling requirements meant Copan could create especially dense systems that took up less space in the data center, and shutting down SATA drives could potentially extend their life expectancy.

But the time required to power up the disks wasn't viable for many applications, and other variations of MAID, also known as intelligent power management, started to emerge. One version parks the disk drive heads while the drives spin at full speed; another simply slows the disk rotation speed; and a third turns off the drive motor.

Beyond SGI/Copan, vendors with disk spin-down technology include EMC Corp., Fujitsu Ltd., Hitachi Data Systems Corp. and Nexsan Corp. Nexsan claims energy savings can range from 20% to 60%, depending on the level of its AutoMAID technology.

"If you can idle a disk and do it without impacting service levels, then you can get a tremendous savings, and the more disk you are able to do that to, the better off you're going to be," said Christine Taylor, an analyst at Hopkinton, Mass.-based Taneja Group. "The trick with MAID is access times."

The original target for Nexsan products was tier 2 applications, but the company claims its customers are now using its systems with all types of applications, including midrange tier 1, according to Randy Chalfant, Nexsan's vice president of strategy.

SGI's Copan storage is designed for online archive and backup, according to Kurt Kuckein, a product line manager at SGI. He said secondary data that is days, weeks or months old is an option, and a tool such as SGI's Data Migration Facility (DMF) can automatically migrate data from primary storage to the Copan system and back on a policy-driven basis. But Copan/MAID is not appropriate for primary data, according to Kuckein.

"A Copan system doesn't make sense until the customer really has more than 100 TB of data that they're looking to store, and even more sense when we're starting to talk about the 300 TB range and getting larger," Kuckein said.

MAID technology 'overrated' according to FMI

Friedrich Miescher Institute (FMI) for Biomedical Research in Basel, Switzerland, learned the hard way about MAID's drawbacks. FMI uses the Copan Revolution 300T virtual tape library (VTL) to store large sets of DNA sequencing data and microscopy images. FMI fronts the system with Sun Microsystems Inc.'s SAM-FS (now owned by Oracle Corp.) highly scalable clustered file system to create a "bottomless NAS," according to Dean Flanders, head of informatics at FMI.

Because of the space and cooling constraints at its main data center, energy efficiency was high on FMI's priority list during the product evaluation stage. The density of the Copan VTL was appealing, allowing FMI to store 40 TB in a 3U enclosure. The dense storage, coupled with virtualized blade servers, even proved helpful on one occasion earlier this year when FMI's cooling unit failed. FMI simply installed a small room air conditioner and opened the door while waiting for the fix.

Copan's MAID technology was also enticing, with its potential to reduce power consumption. But FMI gradually found the power savings didn't compensate for the 16 seconds of disk spin-up time and the periodic time-outs that scientists experienced when copying large amounts of files. Flanders said he fields two or three complaints each month, and worries about the ones that might not reach him.

"I would not recommend using MAID for a file system," said Flanders. "It's perfectly fine for backups or off-line data."

FMI recently purchased a Nexsan Corp. SATABeast, with a 60-drive extension tray, to eliminate the spin-up latency, better meet its scalability needs and afford greater flexibility to use different types of drives, such as SAS disks and solid-state storage. FMI plans to gradually migrate off the Copan VTL to the Nexsan storage as its data footprint continues to climb 25% per year.

Flanders said even though the Copan VTL consumes less power than the Nexsan SATABeast, it's not a significant enough difference given that both consume considerably less than a traditional disk array. Flanders estimated that he will get more than 50% power savings with the Nexsan storage over traditional disk trays.

FMI still plans to use MAID technology, but this time, it won't spin down the disks completely as it did with its Copan system. Instead, Flanders said, FMI will use a MAID level that parks the heads and leaves the disks spinning, even though it will not save as much power as its Copan system did.

"MAID is overrated," Flanders said. "The majority of the power consumption savings is not from spinning down the disks. It's from using power-efficient disks and fewer power supplies."

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