Limitations of solid state disks

Longevity of magnetic storage is rated in mean time between failures (MTBF), a figure that often exceeds one million hours of continuous usage. Western Digital’s prestigious Raptor 10k line offers a 1.2 million hour MTBF rating good for almost 137 years of operation. Though the MTBF rating is egregious in that it does not factor an irreparable end of product life, it is nevertheless a testament to the relative reliability of mechanical drives.

We know that mechanical drives rarely live a decade on from their purchase, much less a century, yet people are comfortable with their volatility because their date of death remains ambiguous. In opposition, the life of a solid state disk is not only clearly limited, but touted as a feature when it improves. Intel’s recent decision to offer solid state disks wowed the market with the promise that the drive could withstand up to 100,000 write cycles.

The write cycle, or the number of times a flash block may be erased and reliably programmed, is taxing for a flash drive. Pouring more than ten volts of electricity through such small and sensitive components takes a toll on the cells and their materials to such an extent that they simply wear out in the end. No longer capable of reliably capturing electrons through the Fowler-Nordheim process, the drive and its data degrade into disrepair.

In order to combat this effect, solid state disks come with a feature called “wear leveling” which intentionally distributes data erratically across the drive to assure that no block is receiving undue usage. Wear leveling and write amplification are just two parts of the bigger host of technologies that are ensuring the continued longevity of flash devices. While 100,000 cycles seems slight, it’s more than 100GiB of new information written to the disk every day for five years before approaching failure. The average lifetime of the SSD is indeed longer than that of a conventional drive, a testament to the power of solid state.

Not all are created equal

The conventional hard drive’s speed is tied very closely to its revolutions per minute (RPMs). Drives like the Western Digital Raptor series excel because their rotational velocity of 10,000 RPMs is almost fifty percent faster than the more conventional 7200 RPM drive. Some drives, particularly in notebooks, may be as debilitatingly low as 3200 RPMs.

Amongst flash drives, there is a similar distinction that is neither as tangible nor as linear: the cell type. Today’s NAND cell can be a single level cell (SLC) or a multi-level cell (MLC). Recall that the state of a NAND cell is determined by the strength of the charge captured in the floating gate. In single level cells there is a single voltage threshold that determines if the cell is programmed as a zero or a one. Multi-level cells have multiple thresholds allowing it to capture two bits of information.

The capacity of an MLC SSD can be up to twice that of an SLC drive that is otherwise identical and equipped with the same number of chips. At up to 250GiB, MLC drives can be spacious, but they can be on the order of two or three times slower than their premium SLC brethren. While exceptional performance from an MLC drive is not out of the question, it is important to identify the role the drive will play prior to purchase.

Winners and losers

As solid state disks grow in popularity, there is great opportunity for new growth in a storage industry that has typically suffered from razor-thin profit margins. The turgid but deliberate pace of conventional drive capacity has left a wash of similar products and bored consumers. In spite of NAS boxes and stylish portable storage, these technologies have only marginally increased the profits of their respective manufacturers.

Flash-based hard drives offer a chance for existing hard drive manufacturers to rebound in a market increasingly condensing under the Seagate brand. Though only a small fraction of the SSD’s cost is returned in profit, a prolonged spike in sales volume will do much to reinvigorate an ailing industry. Because not all flash cells or other internal components are created equally, manufacturers will also have new opportunities to offer premium products.

The solid state market is going to be one that’s vastly different from the conventional drive market. As almost every drive manufacturer runs its own plant, traditional drive companies are saddled with the intense cost of labor and a ballooning capital.  The barrier to entry is much lower for solid state disks given that flash chips are produced in volume by only a handful of manufacturers. This means that a smaller company has the opportunity to purchase a stock of chips and support electronics and assemble it in a much smaller workplace that requires fewer employees. Names like Hama, Memoright and Mtron — names that few enthusiasts have ever heard of — are exploding into household names thanks to the low cost of flash disk production.

But not all companies are prepared to win in the arrival of the solid state disk. Companies that make a living off of managing and addressing issues with conventional drives may be driven to other industries, if not bankrupted by flash. Companies like Diskeeper Corporation have made their living off of industry-leading defragmentation software that would all but ruin a solid state disk. Given that flash devices have a limited number of writes and intentionally fragment their contents, each bit of data moved to a contiguous area just begs for a drive’s early death.

Consider also the popular SpinRite program which has achieved outstanding success in recovering data from mechanical storage. Its crowning feature is the ability to analyze the physical geometry of the hard disk from various angles to reconstruct the contents of information that is unreadable head-on. What happens to SpinRite when there is no grey area between a dead drive and a functional drive?

Winding down

The burgeoning solid state disk industry is a rather different animal from the hard drives we are accustomed to. Even while suffering an unready ecosystem and a consumer base slow to reconcile the new paradigm, flash is already charting an incredible course. Fourteen short months were enough to make a mediocre successor to conventional storage into an undeniable force that will only get better.

As flash disks set to depose magnetic disks to which we owe more than 30 years of storage, the price is falling at a criminal rate. The future’s low prices and ever-increasing performance will open the doors to a whole host of new consumers which will virtually guarantee its success. While new companies and new drive owners delight in the march of progress, we can only wonder what will happen to those firms which depended on a market that may all but evaporate by 2012.