Health-Binning: Maximizing the Performance and the Endurance of Consumer-Level NAND Flash

Abstract

In recent years, the adoption of NAND flash in enterprise storage systems has been progressing rapidly. Todays all-flash storage arrays exhibit excellent I/O throughput, latency, storage density, and energy efficiency. However, the advancements in NAND technology are driven mostly by the consumer market, which makes NAND flash manufacturers focus primarily on reducing cost ($/GiB) and increasing the storage density by technology node scaling, by increasing the number of bits stored per cell, and by stacking cells vertically (3D-NAND). This comes at the cost of reduced endurance of the raw NAND flash, larger variations across blocks, and longer latencies, especially with extremely high error rates (due to the use of read-retry operations). In this paper, we present Health Binning, a technique that facilitates bringing low-cost consumer-level flash to the quality required for enterprise-level storage systems. Health Binning determines the wear characteristics of each block in the background and uses this information in the data-placement process to map hotter data to healthier blocks and colder data to less healthy blocks. Health Binning significantly improves the endurance and performance of the storage system: It actively narrows the block wear distribution and moves endurance from being dictated by the worst blocks towards to a value corresponding to the average endurance of all blocks, resulting in up to 80% enhanced endurance compared with other wear-leveling schemes. At the same time, the probability of reads with high raw bit error rates (RBER) is reduced, thereby decreasing the number of read-retry operations throughout the device lifetime.