Revive Bad Flash-Memory Pages by HLC Scheme

In recent years, flash memory has been widely used in embedded systems, portable devices, and high-performance storage products due to its nonvolatility, shock resistance, low power consumption, and high performance natures. To reduce the product cost, multi-level-cell (MLC) flash memory has been proposed; compared with the traditional single-level-cell (SLC) flash memory that only stores one bit of data per cell, each MLC cell can store two or more bits of data. Thus MLC can achieve a larger capacity and reduce the cost per unit. However, MLC also suffers from the degradation in both performance and reliability. In this paper, we try to enhance the reliability and reduce the product cost of flash-memory-based solid-state drive (SSD) from a totally different perspective. We propose the half-level-cell (HLC) scheme to manage and reuse the worn-out space in SSD; through our management scheme, the system can treat two bad pages as a normal page without sacrificing performance and reliability. The proposed scheme is purely on software/firmware-level, thus there is no need to change the hardware. The experiment results show that the lifetime of SSD with our proposed HLC scheme can be extended to 50.56% under the Windows workload and up to 65.45% under the multimedia workload. When we apply the HLC scheme to flash-memory cache of hybrid storage systems, the response time can be improved up to 20.57%.

I. Introduction

Since flash memory has advantages in nonvolatility, shock resistance, low power consumption, and high performance, both academia and industry believe that flash-memory-based solid-state drive (SSD) has the potential to be a popular mass storage device in the near future. However, flash memory has the reliability problem. The lifetime of flash memory is limited. When a flash-memory block endured over 3000 program/erase (P/E) cycles, data access of pages in the block might suffer from frequent read/write errors. A flash-memory block is regarded as worn out if data cannot be correctly accessed from it.