Lifetime adaptive ECC in NAND flash page management

Abstract

NAND flash memory has decreasing storage reliability, as the density or program/erase (P/E) cycle increases. To ensure data integrity, error correction codes (ECCs) are widely employed and typically stored in the out-of-band area (OOB) of flash pages. However, the worst-case oriented ECC is largely under-utilized in the early stage (small P/E cycles), and the required ECC redundancy may be too large to fit in OOB in the late stage (high P/E cycles). In this paper, we propose LAE-FTL, which employs a lifetime-adaptive ECC scheme, to improve the performance and lifetime of NAND flash memory. LAE-FTL uses weak ECCs in the early stage and strong ECCs in the late stage to guarantee the storage reliability. Since OOB is large enough to store weak ECCs in the early stage, small and size-incremental codewords are adaptively used to improve data transfer and decoding parallelism. In the late stage, strong ECCs have to be employed and the ECC redundancies become too large to be stored in OOB. Thus, LAE-FTL stores the exceeding ECC redundancies in the data space of flash pages and stores user data in a cross-page fashion. Finally, our trace-driven simulation results show that LAE-FTL improves the read performance by up to 63.42%, compared to the worst-case oriented ECC scheme in the early stage, and significantly improve the storage reliability at low cost in the late stage.