JSAC Special Issue, 2016.
Yixin Luo, Saugata Ghose, Yu Cai, Erich F. Haratsch, Onur Mutlu
Carnegie Mellon University
NAND flash memory is a widely-used storage medium that can be treated as a noisy channel. Each flash memory cell stores data as the threshold voltage of a floating gate transistor. The threshold voltage can shift as a result of various types of circuit-level noise, introducing errors when data is read from the channel and ultimately reducing flash lifetime. An accurate model of the threshold voltage distribution across flash cells can enable mechanisms within the flash controller that improve channel reliability and device lifetime. Unfortunately, existing threshold voltage distribution models are either not accurate enough or have high computational complexity, which makes them unsuitable for online implementation within the controller.
We propose a new, low-complexity flash memory model, built upon a modified version of the Student’s t-distribution and the power law, that captures the threshold voltage distribution and predicts future distribution shifts as wear increases. Using our experimental characterization of state-of-the-art 1X-nm (i.e., 15– 19nm) MLC NAND flash chips, we show that our model is highly accurate (with an average modeling error of 0.68%), and also simple to compute within the flash controller (requiring 4.41x less computation time than the most accurate prior model, with negligible decrease in accuracy). Our model also predicts future threshold voltage distribution shifts with a 2.72% modeling error. We demonstrate several example applications of our model in the flash controller, which improve flash channel reliability significantly, including a new mechanism to predict the remaining lifetime of a flash device. Our evaluations for two of these applications show that our model (1) helps improve flash memory lifetime by 48.9%, and/or (2) enables the flash device to safely sustain 69.9% more write operations than manufacturer specifications. We hope and believe that the analyses and models developed in this paper can inspire other novel approaches to flash memory reliability and modeling.
FULL PAPER: pdf