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SanDisk to set standards for SSDs

Posted: 28 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:solid-state drives? flash? storage? NAND?

SanDisk is set to announce a metric that reduces to a single number the expected lifetime of a flash disk, hoping the industry will adopt its Longterm Data Endurance (LDE) approach as a de facto standard for solid-state drives (SSDs).

All sides agree the relatively low number of write cycles on flash memory is a limiting factor for the adoption of flash drives that are just beginning to gain market traction. The industry currently lacks a standard measure for endurance of the devices at the drive level, an issue that a subcommittee announced recently and by which the JEDEC standards group said it aims to address.

"I am told the SanDisk approach is aligned with our direction in JEDEC," said Steffen Hellmold, manager in Seagate Technology's server group, who was aware of LDE but had not yet been briefed on it.

Creation of test standards
Seagate, the world's largest HDD maker, has announced it will start shipping flash drives, also called SSDs in early 2009, initially looking into products for servers. The company is leading the new JEDEC effort that seeks to create a wide array of test standards for flash drives, including metrics for endurance, reliability, performance and length of data storage.

"We need these specifications now so developing them is needed as soon as possible," said Alvin Cox, standards manager, Seagate, who co-chairs the JEDEC JC-64.8 group, launched quietly late last year. "Without test standards people can make claims, even truthful ones, but they may not work in your application," he added.

The JEDEC group is currently reviewing the various uses of flash drives in computing to better identify what test standards are needed to be set.

The limited number of write cycles for flash drives is not significant for PC users whose applications typically have a relatively low number of rewrites in low capacity drives used over the short lifespan of consumer products. However, the issue will be critical for high-capacity SSDs that vendors such as Seagate hope to sell for server applications such as caching database transactions or speeding searches on a Web server.

Vendors generally know how to measure the number of so-called program write/erase cycles for a single NAND flash memory chip used on a flash drive. What has yet to be determined is how to make the same measurements on an SSD, given drive makers use a wide variety of controllers, each with its own scheme for optimizing write endurance.

Getting to JEDEC membership
"The translation of chip-level endurance metric to a drive-level metric requires some work," said Hellmold.

"This is a good time to get involved with the JEDEC subcommittee and still have a voice in what we define," Cox said. "The group hopes with its new test specifications you will be able to determine on a level playing field the differences between various SSDs as well as differences between solid state and hard drives," he added.

Meanwhile, SanDisk claims it has a key piece of the solution with LDE, a measure of the total number of Terabytes written (TBW) a solid-state drive likely can handle. "Our customers want a gas gauge to measure endurance in real time," said Don Barnetson, senior director, SSD marketing, SanDisk.

SanDisk is leveraging in part the work of the Business Applications Performance Corp. that sets performance standards in computing. The group has defined a typical memory-write profile for a professional PC user at 87Gbytes/week.

It's unclear exactly how SanDisk arrives at its TBW measures and how readily it applies to other drive makers using different controllers. Barnetson said SanDisk has tested its approach in its labs and shared the results with PC and OS makers under nondisclosure.

Standing still
So far SanDisk has no supporters publicly committed to the concept. "However, everybody said many like the concept of an endurance gas gauge," Barnetson said.

"We want to make this a standard in the industry," he added. "We are trying to figure out what might be the appropriate committee to standardize everything. JEDEC is a good candidate," he noted.

"It's a great step in the right direction of characterizing drives," said Bob Merritt, memory analyst, Semico Research. "We are all aware of this issue of the lack of endurance metric for flash drives. If this will not be resolved, this can create some problems over the next couple of years," he stressed.

- Rick Merritt
EE Times

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