Solid-state disk lackluster for laptops, PCs

Laptops, desktops won't see a cost/benefit advantage in SSD for about two years

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SSD durability, reliability

Experts also agree that a new standard needs to be established for measuring SSD reliability so the actual life span of flash memory can be more clearly defined.

"There are over 80 companies making SSDs and the quality is all over the map," Gartner's Unsworth says.

Western Digital's SSD drives claim a MTBF (mean time between failure) of 1.4 million hours. Intel claims its new Extreme SSD drives have a MTBF of 1.2 million hours.

However, experts say MTBF is an inaccurate and subjective way to measure drive reliability, because different applications put different amounts of wear on drives. A better measurement to determine longevity may be how many write cycles a drive can handle, or how many times you can write and erase data.

Although SSD does have a durability advantage over hard disk because there are no moving parts to break, the life span of flash memory varies greatly, depending on a number of factors.

For one thing, it matters whether the SSD drive uses SLC or MLC memory. SLC generally endures up to 100,000 write cycles or writes per cell, while MLC can endure anywhere from 1,000 to 10,000 writes before it begins to fail, according to Fujitsu's Hagberg. For its part, Western Digital's laptop hard-disk drive boasts up to 600,000 write cycles.

But SLC also costs about twice as much as MLC to produce, Hagberg says.

Some MLC memory, however, can be faster than SLC memory based on write-amplification correction software, which can boost raw flash chip performance but degrade longevity. With software enhancements, MLC can exceed SLC performance, but at its core, it's still MLC memory, which means its life span is greatly reduced because cells store more data more often. In fact, generally speaking, the higher performance in an SSD drive, the longer life it will have because of better drive efficiency.

But the myth that SSD drives can attain the same longevity as hard disk drives is true only in enterprise-class devices that have wear-leveling software in the drive's controller. This evenly distributes data across the device to ensure that cells do not wear out prematurely.

Hagberg says his company does not plan to launch any SSD drive products over the next two years because the value proposition of the technology is not compelling enough and won't be until technology breakthroughs change SSD's performance and reliability, as long as two years from now.

Still, in less than 10 years NAND flash sales have grown from insignificant numbers to become a $16 billion market, according to Objective Analysis. "This product, the fastest-growing technology in the history of the semiconductor market, is poised to displace DRAM as the leading semiconductor memory," Handy stated in a recent report.

Gartner expects the low-cost SSD category to grow from 635,000 units in 2007 to over 33 million units in 2012. That represents a five-year compound growth rate of 117%, Unsworth says.

For today, however, other than high-end storage arrays and servers, the only other place SSD will see marked growth is in handheld devices -- because their main purpose is reading data. SSDs are also expected to do relatively well in ultraportable laptops and high-end laptops such as Apple Inc.'s MacBook Air, Avian Securities' Cohen says, and only because "it's cool," and not because of any performance advantage.

Copyright © 2008 IDG Communications, Inc.

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