Diversifying wear index for MLC NAND flash memory to extend the lifetime of SSDs

NAND flash-based solid state drives (SSDs) are replacing magnetic disks because of their fast random access performance, shock resistance, and low power consumption. However, the number of program and erase cycles that can be performed on NAND flash is limited. To overcome this limitation, SSDs require a sophisticated wear-leveling algorithm which distributes program/erase cycles evenly across all flash blocks. While most of existing wear-leveling algorithms are only based on the erase counts of flash blocks, our empirical study indicates that the erase count alone is not a good wear index which tells us how much a flash block is worn out. This paper proposes a new wear index for MLC NAND flash memory which takes into account more diverse properties of NAND flash memory. To show the effectiveness of the proposed wear index, we also develop a wear-leveling algorithm, named Equalizer. In our evaluation with three realistic workloads, Equalizer based on the proposed wear index improves the effective lifetime of SSDs by up to 145% compared to the existing wear-leveling technique based on the erase count.