Substantially reduced lifetimes are becoming a critical issue in NAND flash memory with the advent of multi-level cell and triple-level cell flash memory. Researchers discovered that heating can cause worn-out NAND flash cells to become reusable and greatly extend the lifetime of flash memory cells. However, the heating process consumes a substantial amount of power, and some fundamental changes are required for existing NAND flash management techniques. In particular, all existing wear-leveling techniques are based on the principle of evenly distributing writes and erases. For self-healing NAND flash, this may cause NAND flash cells to be worn out in a short period of time. Moreover, frequently healing these cells may drain the energy quickly in battery-driven mobile devices, which is defined as the concentrated heating problem. In this paper, we propose a novel wear-leveling scheme called DHeating (Dispersed Heating) to address the problem. In DHeating, rather than evenly distributing writes and erases over a time period, write and erase operations are scheduled on a small number of flash memory cells at a time, so that these cells can be worn out and healed much earlier than other cells. In this way, we can avoid quick energy depletion caused by concentrated heating. In addition, the heating process takes several seconds and has become the new performance bottleneck. In order to address this issue, we propose a lazy heating repair scheme. The lazy heating repair scheme can ease the long time delays caused by the heating via delaying the heating operation and using the system idle time to repair. Furthermore, the flash memory's reliability becomes worse with the flash memory cells reaching the excepted worn-out time. We propose an early heating strategy to solve the reliability problem. With the extended lifetime provided by self-healing, we can trade some lifetimes for reliability. The idea is to start the healing process earlier than the expected worn-out time. We evaluate our scheme based on an embedded platform. The experimental results show that the proposed scheme can effectively prolong the consecutive heating time interval, alleviate the long time delays caused by the heating, and enhance the reliability for self-healing flash memory.
['Renhai Chen', 'Yi Wang', 'Duo Liu', 'Zili Shao', 'Song Jiang']
Heating Dispersal for Self-Healing NAND Flash Memory