Reliability and performance limitations of NAND flash-based SSDs in demanding enterprise applications will be addressed in 2010 by phase-change memory, according to Ed Doller, CTO of Numonyx.
Higher-end applications like SSDs also use NAND flash, but these applications impose quite different usage conditions and requirements. Standards committees are currently debating testing requirements for enterprise, and whether the use model is 100s of gigabytes written per day or 1,000s of gigabytes per day.
Others argue that the speeds to storage are increasing so fast that even at approximately 5 percent bandwidth utilization you can easily get to 10,000s of gigabytes per day. This is a far cry from MP3 players. A technology that may have been perfect for an MP3 player is going to impose significant limitations in an enterprise-class server.
So far, the cost of NAND flash technology has been driven to extremely low levels through the use of leading edge lithography, but it remains to be seen how much lower it can go. That is why many are looking for alternatives.
One alternative that holds tremendous promise is phase-change memory (PCM). Much has been said and written about PCM, but what is interesting about PCM is that retention is decoupled from endurance. This means that if we cycle a PCM memory device a million times or just one time, the data retention will be identical. To give you some numbers, while NAND endurance capability is reportedly on a decreasing trend, PCM endurance capability is on an increasing trend, with capability in the range of millions to 100s of millions of cycles. That characteristic provides some relief to the most strict-use models.
Another phenomenon that makes it easier to use PCM in system-level designs: when we see a failure, it always occurs during a write. So if we are writing data to the device and a write-verify shows that the data is not there, the data can be immediately written again to another location. PCM is not plagued by intrinsic read-disturb mechanisms, as NAND has been.
Because of the exceptional reliability attributes of PCM, we will see this technology adopted first in applications with the highest most critical requirements.
In addition, the PCM performance characteristics are far faster than NAND as well. At a few orders of magnitude faster latency, system designers can focus their attention on using PCM to extend on the core performance value proposition of SSDs in the enterprise. Also, since PCM has no erase, sectors can be directly overwritten, thereby enabling consistency in seek latency across a much wider range of use conditions.
The good news is that there is hope and a very promising alternative that should allow the industry to continue the transition from HDDs to SSDs with confidence. However, we see many of these issues facing NAND technology in SSDs coming to light in 2010 as designers begin to plan for 2011 and beyond.
This article has appeared on EE Times India, a sister publication to EE Times
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