An Adaptive Idle-Time Exploiting Method for Low Latency NAND Flash-Based Storage Devices

The market share of NAND flash-based storage devices (NFSDs) has rapidly grown in recent years since many characteristics, such as non-volatility, low latency, and high reliability, meet the requirements for various types of storage devices. However, the unique characteristic of NAND flash memories (NFMs), erase-before-write, causes problems for NFSDs from a performance perspective. Specifically, performance degradation is incurred by extra operations that serve to hide the bad characteristics of NFMs. In order to resolve this problem, many attractive methods have been proposed. Various algorithms for flash translation layers (FTLs) are representative methods that provide space redundancy to NFSDs for better performance. However, the amount of space redundancy is limited by the capacity of NFMs and thus, space redundancy is still insufficient for improving the performance of NFSDs. Consequently, a new type of redundancy, termed temporal redundancy, has recently been introduced for NFSDs. More precisely, the idleness of NFSDs is exploited so as to precede extra operations for NFSDs while minimizing the overhead of extra operations. In this paper, we propose an adaptive time-out method based on the Hidden-Markov Model (HMM) to efficiently utilize idle periods. In addition, we also suggest a simple scheduling scheme for extra operations that can be customized for general FTLs. The experimental results demonstrate that the proposed method yields performance improvements in terms of average write latency and peak latency, 74% and 76% better than the existing method, respectively, and approaching within average 9% and 5% of the optimal case, respectively.