How Long Does NAND Flash Memory Last?

For the past three years, consulting firms and others have raised questions about the ability of NAND flash memory to handle large number of "writes" over time, more commonly known as the NAND write limitation issue. These questions raise concerns about the reliability, cost and durability of flash memory and solid state technology in general as it becomes more popular for enterprise systems.

But is there really a write problem that limits NAND in enterprise computing? This hasn't been a major problem in consumer devices like cellphones and digital music players that have been the largest market for flash memory to date. But as NAND flash makes its way into more enterprise storage products, these issues grow in importance.

"Write limitations in NAND are not really an Achilles' heel to end users," says Wayne Adams, chair of the Storage Networking Industry Association (SNIA) and an executive with EMC Corp. (NYSE: EMC) "When you buy a car, it is optimized for a certain oil-fuel mix, but you don't know all the engineering that went into the product to get it to perform. What's going on with NAND is analogous. Engineering algorithms are being applied to improve NAND write performance in the same way that similar technology is employed in hard drives to address their inherent limitations in moving data around. There are always certain parts of media that are ineffective, and we engineer to overcome those limitations."

The current thinking is that, natively, single level cell NAND (SLC) generally supports 100,000 writes, and multiple-level cell NAND (MLC) supports approximately one tenth of the writes that SLC can achieve.

Through a variety of wear-leveling algorithms intended to "spread the wear" over a NAND device, manufacturers incorporating NAND in their products engineer solutions for the write deficiency, extending the lifetime of NAND to the three- to five-year life cycle that hard drives enjoy -- and that IT decision makers look for because it matches their three- to five-year asset depreciation cycles.These life-extending algorithms vary by manufacturer and application, but the overall principle of wear-leveling is to ensure uniform use of NAND device by always directing writes to those NAND blocks with the least wear.

Micron Technology Inc. (Nasdaq: MU) provides the example of a NAND device with 4,065 total blocks and 2.5 percent allowable bad blocks, updating three 50-block files at a rate of one file each 10 minutes, with the NAND host using the same 200 physical blocks for updates.

Without wear leveling, 10,000 cycles x 200 reused blocks/50 blocks per file x 6 files per hour x 24 hours per day equals around 278 days or less than a one-year lifespan for the NAND device.

With wear leveling, 10,000 cycles x 4,096 evenly used blocks/50 blocks per file x 6 files per hour x 24 hours per day equals around 5,689 days or a lifespan of more than 10 years for the NAND device.

"It's really a characteristic of NAND flash technology that we need to use wear-leveling for best performance and extended lifetime," says Phil Mills, chair of SNIA's Solid State Storage Initiative and a senior member of IBM Corp. (NYSE: IBM)'s technical staff. "Wear-leveling algorithms were designed as engineering workarounds for NAND's limitations with write/erase cycles, with the goal of putting NAND on a par with hard drive technology in terms of longevity."Mills maintains that the write idiosyncrasies of NAND are no different from the inherent limitations of other technologies. "If you take a hard drive with a life expectancy of three to five years, it's the mechanics of the drive that usually fail in the form of a head crash where the head gets so close to the media that it touches it and destroys the data," he says. "These kinds of failures happen unpredictably, whereas with NAND SSDs, their composition is entirely electrical and semiconductor. They present a different kind of failure mechanism where breakdowns begin to appear after you do so many writes and begin to see increases in failures of blocks of NAND, which allows you to take action before total failure."

One of the most active areas of NAND research is in multi-level cell NAND, which offers far greater density than its SLC counterpart, which is used today in enterprise computing. MLC offers even greater opportunity to enterprises than SLC, but its use is hindered by very low write capability. "In essence, SLC is first generation NAND flash with one bit per cell," says Mills. "With MLC, you have densities of two, three or more bits per cell. The problem with reliability is that as you go to the higher density, the number of writes per bit that you can do shrinks. What we are working on now is to figure out a way that we can utilize MLC and get enterprise reliability, at the same time that we drive the costs down."

It is higher performance and lower cost in configuration (where large numbers of hard drives can be replaced with fewer SSDs) that is pushing NAND Flash and SSDs. NAND SSD manufacturers have the immediate goals of making their systems equivalent to hard drives in endurance, which is where the write wear-leveling algorithms come in. These algorithms have given NAND Flash and SSD the three-to five year life cycle endurance, and have essentially rendered an initial write limitation (at least for SLCs) a non-issue. But the quest to overcome MLC write limits continues.

"As each of these technologies come into play, each finds its price/performance point and the role that it plays," says Adams. "We are likely to see these technologies incorporated into enterprise life cycle management that features tiers of storage with performance characteristics that are most amenable to SSDs or to hard drives."

Many major tech vendors are devoting a lot of resources to developing better and less expensive NAND technology as they expect the market to continue growing. Major players include Micron, Intel Corp. (Nasdaq: INTC), Samsung Corp. , ST Electronics, SanDisk Corp. (Nasdaq: SNDK), and Toshiba Corp. (Tokyo: 6502) Research firm Gartner Inc. originally predicted the overall NAND market would grow 30 percent this year, but it has cut that forecast in half because of a softening market and an oversupply of product. Analysts estimate the overall consumer and business market for NAND at around $50 billion to $60 billion annually and say that it will stay at that level through 2012 as declining prices offset growing demand.Tech vendors also are working on a number of other techniques to extend the life of SSDs. Micron reported this week that it and Sun Microsystems Inc. (Nasdaq: JAVA) have developed SLC enterprise NAND technology that dramatically extends the lifespan of flash-based storage for enterprise applications. The new technology allows 1 million write cycles and offers the highest write/erase cycling capability of any NAND technology available on the market, the company said. Details of the technology were not immediately available.