CR5M: A mirroring-powered channel-RAID5 architecture for an SSD

Abstract

Manufacturers are continuously pushing NAND flash memory into smaller geometries and enforce each cell to store multiple bits in order to largely reduce its cost. Unfortunately, these scaling down techniques inherently degrade the endurance and reliability of flash memory. As a result, permanent errors such as block or die failures could occur with a higher possibility. While most transient errors like programming errors can be fixed by an ECC (error correction code) scheme, rectifying permanent errors requires a data redundancy mechanism like RAID (redundant array of independent disks) in a single SSD where multiple channels work in parallel. To enhance the reliability of a solid-state drive (SSD) while maintaining its performance, we first implement several common RAID structures in the channel level of a single SSD to understand their impact on an SSD's performance. Next, we propose a new data redundancy architecture called CR5M (Channel-RAID5 with Mirroring), which can be applied to one SSD for mission-critical applications. CR5M utilizes hidden mirror chips to accelerate the performance of small writes. Finally, we conduct extensive simulations using real-world traces and synthetic benchmarks on a validated simulator to evaluate CR5M. Experimental results demonstrate that compared with CR5 (Channel-RAID5) CR5M decreases mean response time by up to 25.8%. Besides, it reduces the average writes per channel by up to 23.6%.