FlexLevel NAND Flash Storage System Design to Reduce LDPC Latency

Abstract

Aggressive technology scaling and adoption of multilevel-cell technique lead to progressive increase of bit error rate (BER) of NAND flash memory. Consequently, conventional error correction code is not adequate to guarantee system reliability. As an alternative, low density parity check (LDPC) code is introduced to provide more powerful error correction capability. However, to achieve better performance, LDPC code demands extra memory sensing operations and more data transfer cycles, directly leading to longer read latency. To achieve both system reliability and read efficiency, we propose the FlexLevel NAND flash storage system design in this paper. FlexLevel consists of two levels of optimization: 1) LevelAdjust and 2) AccessEval. At device level, the LevelAdjust technique is proposed to reduce BER by broadening noise margin via threshold voltage level reduction. With LevelAdjust, BER is greatly reduced and no extra sensing levels are required to protect data integrity. Hence, read performance is improved. However, while LevelAdjust can improve system reliability and read performance, it causes density loss. To balance read performance improvement and density loss, we propose the AccessEval technique at system level. AccessEval identifies data with high LDPC overhead and only applies LevelAdjust technique to these data. The experimental results show that compared with the best existing works, the proposed design can achieve up to 11% read speedup with negligible density loss.