DHeating: Dispersed heating repair for self-healing NAND flash memory

Short lifetimes are becoming a critical issue in NAND flash memory with the advent of multi-level cell and triple-level cell flash memory. Researchers at Macronix have recently discovered that heating can cause worn-out NAND flash cells to become reusable and greatly prolong the lifetime of flash memory cells. However, the heating process consumes a substantial amount of power. This means that some fundamental changes are required if existing NAND flash management techniques are to be applied in self-healing NAND flash memory. In particular, all existing wear-leveling techniques are based on the principle of evenly distributing writes and erases. This causes NAND flash cells tend to wear out in a short time period. Moreover, healing these cells in a concentrated manner may cause power outages in mobile devices. In this paper, we propose for the first time a new wear-leveling scheme called DHeating (Dispersed Heating) to solve the concentrated heating problem in self-healing flash memory. In DHeating, rather than evenly distributing writes and erases over a time period, write and erase operations are concentrated on a small portion of flash memory cells, so that these cells can be worn-out and healed by heating first. In this way, we can disperse healing to avoid the problem of concentrated power usage caused by heating. Furthermore, with the very long lifetime that results from self-healing, we can sacrifice lifetime for reliability. Therefore, we propose an early heating strategy to solve the reliability problem caused by concentrated heating. The idea is to start the healing process earlier by heating NAND flash cells before their expected endurance. We evaluate our scheme based on a real embedded platform. The experimental results show that our scheme can effectively solve the concentrated heating problem.