Solid state disk life and performance varies widely

Rich Coulson, a senior fellow at Intel responsible for the conception, architecture and development of flash memory enhancements, says solid state disks (SSDs) are about to usher in a new phase in drive performance in the enterprise for high I/O applications -- such as transactional databases -- and in the consumer market for mobile devices.

But along with vastly faster, more energy efficient and rugged drives comes a familiar warning: caveat emptor.

Speaking at Storage Networking World in Orlando this week, Coulson explained that the longevity and performance of solid state disk varies based on a number of factors, including the quality of the product and the NAND memory type.

Coulson began by explaining that traditional hard disk drives have only increased in performance by about six times over the past 20 years, while CPU performance has grown 100 times.

"When I started at StorageTek in 1983, the drive of the day was the IBM 3350 and it did about 30 random operations per second. Today a 15K rpm drive will do maybe 300 random operations. So that's 10x in my career. What have CPUs done? Three or four thousand X?" he said.

In contrast, solid state disks, which are quickly being adopted for mobile devices such as laptops and hand helds, and are just beginning be used in enterprises, sport about 100 times the IOPS of traditional spinning disk and are about two and a half times more energy efficient. So the use of SSDs can result in significant reduction in system-level TCO as well as greater reliability.

Coulson admitted that to date SSD reliability has been a problem, particularly in notebook PCs where some of the biggest uptick in adoption has been seen. He said write performance has also been markedly poor in the early drives -- in some cases slower than spinning disk. Data transfer speeds can vary as much as five times depending on the type of SSD and lifespan can range from less than 6 months to more than five years.

Coulson dismisses mean time to failure as an accurate metric for determining drive longevity, and instead says good solid state drives should be able to perform 3,000 8KB writes per second for five years. "Good solid state disks have sufficient life spans for enterprise applications," he said.

One series of tests from the Transactional Processing Performance Council showed write transfer rates among SSDs from a variety of vendors ranged from 113/sec to 26/sec and read rates varied from 142/sec to 49/sec, Coulson says.

"Most solid state disks shipping in notebooks today have pretty poor write performance," Coulson said.

Among the factors differentiating NAND memory quality is whether it is single-level cell (SLC) memory or multi-level cell (MLC) memory. SLC stores one bit of data per memory cell while MLC stores two to four bits of data per cell, affording it greater density, but slower data transfer speeds, higher power consumption and shorter lifespan. In general, SLC costs than MLC to manufacture and is sold for about twice as much as MLC memory, Coulson says.

But SLC vs. MLC isn't the only factor differentiating the quality of NAND memory. Some MLC memory is actually faster than SLC memory based on write amplification methods, which can boost raw flash chip performance but degrade longevity.

Generally, the higher performance an SSD drive has the longer life it will have because of better drive efficiency. So if you're looking for longevity, you'll also get greater IOPS along with it, said Coulson, who believes the SSD costs will soon drop opening the market for greater use.

Currently, solid state NAND memory costs about $8 a gigabyte for OEMs compared to well below $1 per gigabyte for hard disk drive storage, but as the mobile market continues to up its use of flash memory over spinning disk, an economy of scale will soon be reached and the price of SSDs will drop by an order of magnitude, Coulson said.

Copyright © 2008 IDG Communications, Inc.

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