Another StorageMojo Best paper, The Bleak Future of NAND Flash Memory, presented at this year’s FAST ’12 conference, quantifies flash’s declining reliability, endurance, and performance as density increases.
Researchers Laura M. Grupp and Steven Swanson from the UCSD Non-volatile Systems Lab and John D. Davis of Microsoft Research collected data from 45 flash chips from 6 manufacturers. Using that empirical data they predict the performance and cost characteristics of future SSDs.
Faster better cheaper or slower worse cheaper?
While NAND flash is produced with semiconductor processes, smaller feature sizes don’t lead to faster performance or greater reliability. As NAND features shrink, so do the number of trapped electrons that store information.
Figures of merit
The research found that performance, program/erase endurance, energy efficiency, and data retention time all got worse with feature shrink.
Based on past performance, the team derived equations to describe how changes in feature size have affected key specs. They looked at SLC, MLC and TLC and feature sizes scaled from 72 nm to 6.5 nm (the consensus smallest feature size published in the International Technology Roadmap for Semiconductors (ITRS0), and assumed a fixed silicon budget for flash storage.
- Latency. MLC write latency will double over time. Triple-level cell writes will grow to over 2.5MS, noticably reducing its performance advantage over disk writes.
- Bandwidth. Small – 512B – read bandwidth and all writes decline by up to 50% over time. The impact is greatest on high-performance SLC flash.
- IOPS. MLC flash I/O rates will drop almost in half.
Flash may be the new disk in a few years.
The StorageMojo take
One important qualifier is that for the purposes of their modeling the team constrained the number of chips in the hypothetical future devices whose performance they predicted. While fine for isolating the impact of future chip shrinks, it ignores the potential of much greater parallelism for managing these changes.
Bandwidth drops by half? Double the number of chips.
But if something can’t go on forever, it won’t. NAND flash will soon enter an end-of-life crisis for computer applications that need performance. That’s why ReRAM (resistance RAM) looks to be a good bet for replacing computer flash – not mobile device flash – over the next decade.
Courteous comments welcome, of course. A version of this post was published on ZDNet last week.