Exploiting process variation for retention induced refresh minimization on flash memory

Solid state drives (SSDs) are becoming the default storage medium with the cost dropping of NAND flash memory. However, the cost dropping driven by the density improvement and technology scaling would bring in new challenges. One challenge is the overwhelmingly decreasing retention time. The duration of time for which the data written in flash memory cells can be read reliably is called retention time. To deal with the decreasing retention time, refresh has been highly recommended. However, refresh will seriously hurt the performance and lifetime, especially at the end life of flash memory. The second challenge is the process variation (PV). Significant PV has been observed in flash memory, which introduces large variations in the endurance of flash blocks. Blocks with high-endurance can provide long retention time, while the retention time is short for low-endurance blocks. Considering these two challenges, a novel refresh minimization scheme is proposed for lifetime and performance improvement. The main idea of the proposed approach is to allocate high-endurance blocks to the data with long retention time requirement in priority. In this way, the refresh operations can be minimized. Implementation and analysis show that the overhead of the proposed work is negligible. Simulation results show that both the lifetime and performance are significantly improved over the state-of-the-art scheme.