LGR

Abstract

NAND flash refresh schemes have been proposed to mitigate retention errors by remapping pages before excessive errors occur in them. However, the refresh operation induces write amplification deteriorating performance and lifetime of NAND flash memory. Moreover, with the emergence of 3D NAND flash memory, new error characteristics, such as layer-to-layer process variation, worsen the efficiency of con- ventional refresh schemes. To cope with these problems, we propose LGR, a Layer Granularity Refresh scheme, to enhance the lifetime of NAND flash memory. Herein, we measure the inefficiency when con- ventional refresh schemes are applied under 3D NAND flash memory. Furthermore, we propose a refresh scheme with layer granularity different from the conventional block granularity. With LGR, the number of valid page copies induced by refresh can be reduced, thus increasing lifetime. Experimental results with real-world workloads show a reduction in valid page copies, improvement in the estimated lifetime, and reduction in the average response time by 29.5%, 15.3%, and 16.0%, respectively.