Dynamic Erase Voltage and Time Scaling for Extending Lifetime of NAND Flash-Based SSDs

The decreasing lifetime of NAND flash memory, as a side effect of recent advanced semiconductor process scaling, is emerging as one of major barriers to the wide adoption of SSDs in high-performance computing systems. In this paper, we propose Dynamic Erase Voltage and Time Scaling (DeVTS), an integrated approach to extend the lifetime (particularly, endurance) of NAND flash memory. DeVTS is motivated by our key observation that erasing a NAND block with a lower voltage or at a slower speed can significantly improve NAND endurance. However, using a lower erase voltage causes adverse side effects on the write performance and retention capability of NAND flash memory. In order to improve NAND endurance without affecting the other NAND requirements, we take advantage of idle times between write requests and variations of the retention requirement when writing data to a NAND block erased with a lower voltage. We have implemented a DeVTS-aware FTL, called dvsFTL, which exploits the tradeoff relationship between the endurance and erase voltages/times by accurately predicting the write performance and retention requirements. Our experimental results show that dvsFTL can improve NAND endurance by 94 percent, on average, over an existing DeVTS-unaware FTL while all the NAND requirements are preserved.