Solid-state disk will go mainstream in 3, 2, 1...

Key Developments

A key way SSD manufacturers such as Intel, Toshiba, Samsung and SanDisk are lowering the price is by moving from single-level cell (SLC) to multilevel cell (MLC) technology. While SLC-based SSDs store one bit of data per memory cell, MLC drives store two, four or more bits per cell. Because this increases capacity per square inch, MLC can offer twice the capacity as SLC at the same cost.

However, it also lowers performance -- SLCs are roughly twice as fast as MLCs, according to Joe Unsworth, an analyst at Gartner Inc. Toshiba's notebook, which uses MLC technology, has write speeds of 40MB/sec. and read speeds of 100MB/sec., although Samsung is claiming higher speeds -- 70MB/sec. write and 100MB/sec. read on its 128GB SSD, also MLC-based. Still, hard drives typically read data at about half this speed.

MLC also reduces SSD reliability by an order of magnitude because the chip wears out faster. To compensate, vendors like Toshiba and Samsung have developed what they call "wear leveling" algorithms, which reside on the controller chip inside the SSD along with the NAND flash chip. Using the algorithm, the controller spreads writes across the drive to avoid wearing out any one portion too quickly. "We're in the first phase of MLC this year, and as time goes by, consumers will see how reliable it is," Chander says.

Manufacturers are also shrinking the size of SSD devices to lower costs and increase capacities. For instance, Intel and Micron's 32GB chip is based on 34-nanometer technology, which increases the density of SSD chips on the wafer -- which in turn lowers cost, Chander explains. According to Intel, the product will enable more cost-effective SSDs, instantly doubling the current storage volume of these devices and driving capacities to beyond 256GB in today's standard, smaller 1.8-in. size. But there is a trade-off, Chander says, because even as you squeeze out costs through miniaturization, the design becomes more complex.

A key development that will increase SSD performance is the release of Open NAND Flash Interface (ONFI) 2.0-compliant parts, according to Stokes. This type of flash has dramatically faster read and write speeds, he says, which will change the performance equation for everything from SSDs in the enterprise and in mobile devices, to consumer products like Apple's iPhone. Intel's new SSDs will be based on the ONFI 2.0 spec, so their transfer rate will be blazing fast, Stokes says.

The spec, released earlier this year, defines a high-speed NAND interface that delivers up to 133MB/sec. compared to the original NAND interface's 50MB/sec., which significantly hampers performance in applications such as SSD.

ONFI 2.0 reduces the time required to transfer data to and from the data buffer by using two techniques, according to the ONFi Working Group. The first is DDR (Double Data Rate) signaling, which transfers data on both the rising and falling edges of the clock signal, doubling the data transmission rate, a technique commonly used in DRAMs. The second is source-synchronous clocks that accurately latch signals, enabling higher frequencies to be realized.