Management of Next-Generation NAND Flash to Achieve Enterprise-Level Endurance and Latency Targets

Abstract

Despite its widespread use in consumer devices and enterprise storage systems, NAND flash faces a growing number of challenges. While technology advances have helped to increase the storage density and reduce costs, they have also led to reduced endurance and larger block variations, which cannot be compensated solely by stronger ECC or read-retry schemes but have to be addressed holistically. Our goal is to enable low-cost NAND flash in enterprise storage for cost efficiency. We present novel flash-management approaches that reduce write amplification, achieve better wear leveling, and enhance endurance without sacrificing performance. We introduce block calibration, a technique to determine optimal read-threshold voltage levels that minimize error rates, and novel garbage-collection as well as data-placement schemes that alleviate the effects of block health variability and show how these techniques complement one another and thereby achieve enterprise storage requirements. By combining the proposed schemes, we improve endurance by up to 15× compared to the baseline endurance of NAND flash without using a stronger ECC scheme. The flash-management algorithms presented herein were designed and implemented in simulators, hardware test platforms, and eventually in the flash controllers of production enterprise all-flash arrays. Their effectiveness has been validated across thousands of customer deployments since 2015.