The bottom line on solid state drives

At the 2008 annual Usenix conference in Boston, Andrew Leung, a computer science researcher at the University of California, presented a finding that just about everyone must have suspected: The majority of an organization’s stored data sits untouched.

Leung was part of a team that studied how information technology vendor NetApp accessed its 22T of corporate data. They found that, in a three-month period, no one accessed 90 percent of the company’s data — not even once.

That statistic would be useful to keep in mind when dealing with a newfangled technology called solid-state drives (SSDs).

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Advocates like to say that we are close to a major turning point in computer storage, and one day, our computers will use SSDs instead of the hard drives we are all familiar with.

"We're entering an inflection point in the industry where solid-state storage has achieved the costs and capacity and performance to enhance computing platforms," said Troy Winslow, marketing manager for Intel's line of recently introduced SSDs.

But though it seems that SSDs are doubling in size and falling in price every 18 months or so, hard-drive manufacturers have kept their products ahead of that curve.

"Yes, the costs for SSDs can come down, but there will also be a premium to solid-state on a dollar-per-gigabyte basis" for some time, said Matt Bryson, a senior research analyst at Avian Securities. "For the foreseeable future, unless the drive-makers have a problem with technology," hard drives will continue to outpace SSDs, at least in terms of cost.

However, many experts see advantages to SSDs. For example, Bryson said the best use of the drives is as a way around performance barriers.

"I can't say we'll ever reach the dollars-per-gigabyte for a hard drive,” Winslow said. “But because of the performance, [ruggedness] and low power usage, we don't feel that the industry needs to match the hard-drive costs to become a mainstream player in the storage space."

Instead of price, advocates emphasize SSDs’ faster read/write times, lower power usage and improved reliability. They say the dollars-per-gigabyte comparison might not always be the best way to determine which kind of drive to choose.

And considering the kind of low data use researchers have found, SSDs could play a pivotal role in speeding access to the data that users work with more frequently.

All digital, all the time

For geeks, there is something inherently appealing about SSDs.

In many ways, the traditional hard drive is a throwback to pre-computer ways of getting things done through mechanical actions. A hard drive is basically a set of disks, much like the vinyl records used to hold music. A hard drive encases a number of platters, each of which is coated with a thin magnetic film. The platters are divided into billions of cells, each of which represent a 1 or 0 in binary data storage and each of which can be magnetized or demagnetized. Data is written to those platters by read/write heads, the equivalent of the needle on a record player.

In contrast, SSDs have no moving parts. They are created in much the same fashion as microprocessors — by etching digital logic gates into silicon through a lithography process. SSDs are typically huge conglomerations of Not-Or or, in most cases, Not-And (NAND) gates.

SSDs work in a way similar to a computer system’s RAM. Unlike RAM, however, SSDs are nonvolatile, meaning they retain their information without a power source, as a thumb drive does.

Invented in Japan in the early 1980s, nonvolatile solid-state memory only recently has come down in price enough to be considered for widespread use. You might have noticed the proliferation of handheld thumb drives in the past few years, which are offered with ever-larger memory. Such drives are becoming large enough, and almost inexpensive enough, to serve as hard drives for computers.

However, they face a tough struggle to keep up with today's dominant hard-drive technology. Each year, new hard drives come out with larger capacities. You can now buy a drive with 2T of storage for less than $300 at Best Buy. That type of drive would have cost thousands of dollars a few years ago. By contrast, that much capacity in solid-state form would cost about $15,000.

Faster is better

So it seems to be a no-brainer to stick with hard drives — and it is, unless you have certain operating characteristics to consider. Performance is one of them. In certain settings, SSDs can outperform hard drives by a wide margin.

"Hard drives are [creating] a huge bottleneck that is choking off our CPUs, and SSDs have the ability to bridge that gap," Winslow said. "The multicore processes just can't get enough data from the memory systems, and they are not fully utilized."

For this market, Intel introduced the Intel X25-E Extreme SSD last fall. The 2.5-inch drives can deliver write speeds of 250 megabits/sec and sequential write speeds as fast as 170 megabits/sec. In terms of performance, the disks can perform as many as 35,000 input/output operations per second, compared with the 300 IOPS typically offered by a high-performance hard drive spinning at 15,000 revolutions per minute. The Intel drives are available with 32G and 64G of capacity, and they consume about 240 milliwatts when in full use. The 32G drives start at $695.

Although undeniably more expensive than hard drives on a straight per-gigabyte basis, SSDs could be cheaper in some settings, Winslow said.

For one, their speed allows organizations to use the whole drive. To reduce write and read times for traditional hard drives, many organizations adopt a practice called short stroking in which only the outer sectors of each drive are filled with data, ensuring that the disks can find and return data quickly.

"Because the disk is rotating around and the mechanical arm is coming from the outside, they put all the data on the outside edge so the mechanical latency to access is reduced," Winslow said.

The downside is that "they are not fully utilizing the capacity of the drives. So in theory, if you had a pure read application, you could replace up to 115 [high-performance] drives with one SSD," he added. "Realistically, since there is a blend of reads and write, you could replace 10 to 35 [15,000 rpm] drives with a single SSD drive."

In other words, for organizations that use short stroking, SSDs could be cheaper than traditional hard drives.

SSDs have other potential benefits in the desktop and laptop PC markets, Winslow said. Most notably, they are more rugged and use less power than traditional hard drives.

Last fall, Intel released SSDs for laptop and desktop machines — the Intel X18-M (1.8-inch) and X25-M (2.5-inch) Mainstream SSDs, respectively. They are available in 80G or 160G capacities. The 80G model starts at $595. The drives draw 150 milliwatts during full operation.

The drives are based on Intel's multilevel cell NAND flash technology. MLC is Intel's spin on the flash memory market, and it allows more than 1 bit of data to reside on a memory cell. The approach can reduce costs and increase a drive’s capacity, but Winslow said it can also slow performance. The company has stuck with single-level cells for its server drives, which makes them faster though not quite as energy efficient.

For laptop PCs, SSDs have some obvious benefits. For example, because they have no moving parts, they can better withstand the rough and tumble life of a laptop computer.

Furthermore, because SSDs require less power to operate, the amount of battery life for a laptop PC equipped with solid-state technology can far outstrip that of a traditional laptop. Winslow said Hewlett-Packard offers laptop PCs that can run for as long as 24 hours on a single battery charge. And SSDs consume little energy when idle. For example, the Intel drives consume 60 milliwatts when not in use.

What's not to like?

If SSDs ever match the price of hard drives, it is a natural assumption that they will become the dominant storage technology. But even if they hit price parity, they still have some drawbacks.

"We feel it is niche at best,” said Don Jeannette, senior product marketing manager at hard-drive manufacturer Fujitsu, which does not offer any SSDs. “We think SSDs will live in conjunction with traditional hard-disk drives for the foreseeable future."

Jeannette said only 3 percent of laptop PCs have SSDs, and the company estimates that the technology’s share of the mobile market might only reach 10 percent to 15 percent by the next decade. Price is the biggest reason for the disparity now, but eventually, hard drives will likely grow much larger in capacity. But how big a hard drive does the average laptop PC need?

For now, the question of size falls in favor of traditional hard drives. The sweet spot for laptop drives is 160G, and 32G for those with SSDs. With today's multimedia needs, that space can get used up pretty quickly, so users are clamoring for bigger hard drives, Jeannette said.

Another issue for SSDs is longevity. Although they have no moving parts, they still have limited life spans of about 10,000 reads and writes per memory cell, Jeannette said.

"If you keep writing bits to the same SSD block, it becomes only a matter of time before that block goes bad and it loses its electronic properties, whereas you can write to a hard-drive track an infinite number of times," Jeannette said, adding that vendors are developing software that disperses the use across all the blocks, thereby spreading out the wear and tear.

The magnitude of the problem is still open to debate. For instance, Intel claims its 32G X25-E model can write as much as 4 petabytes of data in a three-year period, or about 3.7T a day.

Part of the issue is metrics. SSD vendors do not agree on a standard way of measuring mean time between failures. Specifically, there are no models for what constitutes standard use. Does the drive being measured get 3.7T of data a day to write or something less than that?

Those points aside, almost everyone agrees that SSDs will be used in some capacity at almost every data center. Perhaps the most likely scenario is that IT managers will use them to serve the most frequently accessed data and keep the rest of the data in large pools of cheaper hard drives.

"There is a strong belief in the industry that, three or four years down the road, there will be very fast SSDs in front of very slow but very big hard drives," Bryson said.