BPM: A Bad Page Management Strategy for the Lifetime Extension of Flash Memory

The lifetime of NAND flash is highly restricted by bit error rate (BER) which would exponentially increase with the number of program/erase cycles. While the error correcting codes (ECC) can only provide a limited error correction ability to tolerate the bit errors. To face this challenge, a novel bad page management (BPM) strategy is proposed to extend the lifetime of NAND flash based on the experimental observations in our hardware-software co-designed experimental platform. The experimental observations indicate that retention error is the dominant type of NAND flash errors, which is caused by the charge leakage in memory cells over time. The BER distribution of retention error shows distinct variance in different pages. The key idea of BPM is to excavate lifetime potency of each page in a block by introducing the fine granularity bad page management instead of the coarse granularity bad block management. In addition, to balance the lifetime enhancement and the storage capacity degradation, a configurable threshold of bad page management (CT-BPM) strategy is proposed to utilize in the storage capacity highly demanded applications. The experimental results show that BPM can provide dozens of times (about 35 times for 3x-nm NAND flash) average lifetime extension without additional hardware cost, while experiencing at most 5% degradation in writing speed.