| Summary: | 碩士 === 國立臺灣科技大學 === 資訊工程系 === 104 === In recent year, NAND flash memory has been widely used as primary storage medium such as embedded systems, portable devices, and high-performance storage products, the reasons including its non-volatility, lightweight, high performance, low power consumption and shock resistance. As the improvement of manufacturing technology, the cost per unit of NAND flash memory has been fast decrease. However, the program/erase (P/E) cycles of flash memory also keep decreasing since the density of NAND flash memory increasing. Thus, to improve the lifetime of flash memory has become a critical issue in the design of flash-based products. In NAND flash-based solid state driver (SSD), wear leveling is used to prolong the device lifetime. Its algorithm try to distribute the erase to flash block evenly since the flash memory can only endure a limited number of P/E cycles. In this paper, we distribute the bit error rates (BER) instead of P/E cycle. This work is inspired of observation of flash blocks endured the same P/E cycles usually have a different number of error bits. Since the main reason of flash block failure depend on the uncorrectable error in a page with ECC. This paper try to even out the BER based on the identification of number of weak pages in a flash block. The experiments shows that the proposed mechanism can efficiently prolong the lifetime by balancing the weak pages among flash blocks. It improves the lifetime of total device failed by up to 67\% in general workloads. For performance concern, weak-page-based wear leveling bring acceptable performance overhead in read response time. These results shows that weak-page-based wear leveling is better to use for the applications which need to achieving higher reliability.
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