SSDs go mainstream

The market is flooded with options, and the performance we've seen from several of the latest drives in our tests back up some of the claims that SSD supporters have made about the technology's advantages.

Until the advent of SSD, standard PC storage relied on magnetic hard-disk technology, which has numerous moving parts (including a spindle motor, an actuator assembly, and read/write heads that float barely 10 nanometers above the spinning platter surface).

By contrast, SSD storage consists of NAND flash-memory chips. SSD's lack of moving parts gives it an edge over regular drives on multiple levels. First, SSDs are more shock-resistant than magnetic hard-disk drives; SSDs have fewer potential points of mechanical failure, and are able to withstand jostling and sudden impacts.

In pricing, SSD currently has little hope of competing with standard hard drives, and this won't change anytime soon. Expect to pay upward of $2.75 per GB for an SSD, versus about $0.25 per GB for a regular hard drive. Capacity remains relatively limited, too: 256GB is the current high end for mainstream SSDs. Because of that limitation, SSD makers don't target storage-hungry users. If you need high capacity, look to standard hard drives; they can offer more than triple the capacity of SSD at a fraction of the price.

“In the longer run, both magnetic disk and SSD cost per capacity will continue to decline, so magnetic disks will retain a price per capacity advantage. However, the absolute cost for an SSD will continue to decrease, increasing the set of customers for which SSDs have sufficient value to be worth using,” says Stefan Wolfs, SWD Business Development Manager, HP Middle East.

Despite its disadvantages, SSD is breaking into the mainstream due to the proliferation of models and the lower prices that have come in the last year from increased competition and improved production processes. Notably, Intel recently introduced its smaller and less expensive 34nm NAND multilevel cell flash memory.

“Many customer applications today need 10’ or 100’s of HDD’s to meet their performance requirements, but do not need the capacity of these disks. Because of this many disks are under-utilised – typically 20-30% – meaning much disk capacity is paid for and consuming power, but wasted. SSD’s offer significantly increased I/O performance per drive, thus reducing this waste,” says Martyn Molnar Regional Sales Director, NetApp, explaining the adoption of SSDs in the enterprise.

However, this technology is not backed by decades of experience, understanding, and standardized attributes, and will take years to reach such maturity, says Wolfs. “Therefore solid state storage, while already good and useful, will initially find use in the enterprise by those who can navigate these uncertainties and corresponding hype, such as early adopters and those with specific needs uniquely addressed by the technology. These users will naturally want to understand how this technology will affect performance, availability, reliability, and cost of their computer storage.”

SSD pros and cons

Performance sees improvement, too, but the benefits of using an SSD are not apparent across all applications. For now, SSDs force you to accept a trade-off: They offer faster read speeds, but in write speeds they trail 7200-rpm magnetic hard disks (and can even fall short of 5400-rpm hard disks).

Compared with standard hard drives, SSDs are capable of reduced latency, which translates into greater speed in accessing data. For example, Intel says a typical hard-disk drive's latency is 4000 microseconds, while the company's X-25M is rated at 65 microseconds. SSDs have faster seek times than hard-disk drives do, too. Newer drives, such as the X-25M, boost random write performance, which can have a positive impact on system and app responsiveness.

But not all SSDs are created equal. Everything from the source of the NAND flash to the chipsets and controllers to the wear-leveling algorithms used (more on that in a moment) can affect performance. Single-level cell (SLC) flash, for instance, is costlier than multilevel cell (MLC) flash, but it's also capable of greater endurance. Most consumer SSDs today have MLC flash; when drives are significantly pricier or are sold as “enterprise” drives, the reason may be that they have SLC flash.

Although the SSD market is crowded with contenders, only a few companies, such as Intel and Samsung, manufacture the flash memory. They supply the flash–and often the drives themselves–to other vendors, which “rebadge” the drives as their own.

One largely unpublicized, but critical, aspect of SSDs slightly reduces the technology's attractiveness. In comparison with hard-disk platters, NAND flash memory cells can rapidly wear out with use. As a result, SSD makers employ wear-leveling algorithms to make the drive write data evenly across the flash cells. Whether the algorithms are effective in the long run remains to be seen, however. And consumers must accept a manufacturer's word as to how well its algorithm will safeguard their data; users have no way to gauge the drive's actual wear-leveling effectiveness.

Another little-discussed issue: Out of the box, SSDs can offer blazing speed, but over time their performance may degrade, depending on how you use the drive. Unlike with standard drives, with SSDs the sequential or random nature of the writes will affect future performance. Sequential writes generally leave a few large blocks of free space that make recycling, or garbage collection of data, faster. Every operating system, however, performs random writes that users can't control; in random writes, the remaining space is very small, and that causes garbage collection to take a lot of time.

Some manufacturers, Intel among them, estimate the lifetime of an SSD in its specs (Intel says five years). Along with other SSD makers, Intel also uses the same measurement that standard hard-disk drive manufacturers use, referring to the drive's life expectancy in terms of the mean time between failures. Among the SSD drives whose makers list this spec, the typical MTBF is between 1 million and 1.2 million hours, though at least one (Samsung) goes as high as 2 million hours, putting SSD at or above enterprise-class hard-disk drives in reliability, and far above consumer-class hard-disk models; manufacturers don't even list this spec for consumer hard-disk drives.