Computerworld

Affordable SSDs in the offing?

New flash memory controller technology from Silicon Motion promises cheaper, more affordable SSDs

September is usually too early to draw year-end conclusions, but I'll go out on a limb to say that 2008 will be remembered as a key year for SSDs (solid-state drives) -- not so much for sales figures, which won't likely reflect the hype surrounding flash SSDs this year, but for the technology itself, which remains one of the most controversial to hit storage in recent times.

Flash SSDs are based on technology rich with technical details that are generally misunderstood. To clear up some of the uncertainty around flash SSDs, I spoke with Wallace C Kou.

Kou is president and CEO of Silicon Motion, a company based in Taiwan and California, and one of the major players in SSD technology. Due to our busy schedules and distant time zones, Kou and I conducted our interview over e-mail.


Please introduce Silicon Motion to our readers.

Silicon Motion is an innovative technology company that designs, develops, and markets high-performance, low-power semiconductor solutions for the mobile multimedia consumer electronics market. We have three major product lines: mobile storage, mobile communications, and multimedia SoC (system-on-a-chip). Our mobile storage business is composed of microcontrollers used in NAND flash memory storage products such as flash memory cards, USB flash drives, SSDs, embedded flash applications, and card readers. Our mobile communications business is comprised of mobile TV tuners, CDMA RF ICs, and electronic toll collection RF ICs. Our multimedia SoC business is comprised of products that support MP3 players, PC cameras, and embedded graphics applications.

How do you see the flash memory market today?

We believe that NAND supply is still bigger than demand and that the industry's excess supply condition will continue through to the end of 2008. Since overall demand is still soft, the movement of channel inventory is slow, which is affecting companies involved in the NAND food chain.

What market segments are you targeting?

Our three product lines target three markets: solid-state storage devices, principally flash memory cards, USB flash drives, SSDs, and embedded flash devices, mobile TV involving DVB-H, T-DMB, S-DMB, ISDB-T, CMMB, and T-MMB standards, and embedded graphics applications. The end application for approximately two-thirds of our revenue, whether from sales of controllers for flash memory cards or mobile TV ICs, is the large and rapidly growing mobile phone industry.

What is an SSD controller, and how does it work?

A controller plays a very important function in a SSD. It manages the data input/output between a host device and the NAND storage media. It also handles various host protocol, special custom command set, data error detection and correction, as well as NAND flash management, such as bad block management, error handling, wear-leveling, data protection during power off, performance, endurance, and reliability, etc.

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Please explain what a hybrid SSD is.

Hybrid SSD utilizes both SLC and MLC NAND to achieve low cost, high performance, and high endurance. Our hybrid SSD controller can, for example, manage a combination of, say 8GB of SLC and 56GB of MLC in a 64GB device. The controller utilizes our proprietary algorithms to identify frequently accessed and infrequently accessed files and puts the frequently accessed files into the SLC NAND and the infrequently accessed files into the MLC NAND. By using our hybrid SSD controllers, SSD OEMs can achieve endurance and performance similar to an SLC-only SSD, but at the cost close to an MLC-only SSD.

Your company recently announced new SSDs controllers. Please describe their major features.

Our new SSD controllers support SSD OEMs who chose to use only SLC, only MLC, and a mix of SLC and MLC flash. Our controllers provide both advanced 8-bit to 24-bit ECC (error correction code) engines for enhanced reliability and "global wear-leveling," which means wear-leveling on a device basis as opposed to wear-leveling at each individual NAND flash component in the device. Global wear-leveling performs its task by taking the logical block address from the host and randomly mapping it into the physical block address of the bank of NAND flash components. In addition, our new controllers add new data protection technology to prevent data loss due to unexpected power surge or failure.

Two of the new controllers have a PATA (parallel ATA) interface. Isn't that old technology?

In low-cost notebook PCs and embedded system applications, PATA is still popular because it consumes less power and has a cost advantage to the newer SATA interface. It is important for us to provide the various solutions that our customers require.

Can you give an idea of how much the cost of an SSD controller impacts the final product?

The cost of the controller is a very small part of the overall cost of a SSD. The bulk of a SSD cost is from NAND flash. Despite the controller accounting for a relatively small proportion of an overall SSD cost, it plays a disproportionately large role as an enabler. Currently, most SSDs are using SLC NAND flash because of a lack of availability of suitable MLC controller solutions. When a robust MLC controller, such as ours, becomes available, SSD OEMs will be able to reduce the cost of their SSD solutions -- of the same capacity -- by one half to a third, or enable SSD OEMs to double or triple their SSD capacity for the same cost. [Emphasis mine, not Kou's (see below).]

Leading global notebook PC OEMs and consumer electronics OEMs are therefore all looking for better solutions that could enable them to use MLC NAND for SSD. SSD controllers could quickly become a mainstream storage device when robust MLC controllers are available that enable OEMs to use less expense MLC NAND flash. Our MLC controllers for SSDs have just become available for sampling.

The wear-leveling logic seems to be a very critical part of the controller. Is that part stable or still work in progress?

NAND is rapidly transitioning to 40nm and 30nm process nodes. Flash makers can significantly reduce their cost by moving into these finer geometries, but achieve this benefit at a heavy cost. Endurance for these newer products is significantly reduced when compared to products based on 60nm and 70nm process technologies. The controller therefore has to provide significantly more advanced and robust wear-leveling algorithms, by randomly and equally accessing every physical block in all the individual NAND components in that SSD device, to enhance SSD endurance. We have already achieved some excellent results with our global wear-leveling approach. We will continue to develop newer and more advanced wear-leveling techniques to meet upcoming NAND flash technological challenges.


I emphasize the above statement about cost reduction because the promise of getting three times the bang for the buck is not something we hear often, in storage or other areas. If Kou's prediction is correct, the price of flash SSDs will indeed soon become much more competitive with that of traditional spinning drives.