Overclocking NAND Flash Memory I/O Link in LDPC-Based SSDs

Abstract

Due to the increasingly significant process variation and gradual Flash memory cell wear-out, the worst-case-oriented error correction code (ECC) in solid-state drives (SSDs) is mostly underutilized throughout the entire lifetime. Error-prone overclocking of Flash memory chip I/O links can trade such ECC underutilization for opportunistically improving SSD speed performance, and its effectiveness strongly depends on how well the ECC decoding can handle the overclocking-induced I/O link errors. As SSDs are quickly adopting low-density parity-check (LDPC) code, this brief concerns LPDC-based overclocked SSDs. Experiments with 20-nm NAND Flash memory chips reveal unique bit error characteristics of the overclocked I/O link, based upon which this brief develops solutions that can leverage the error characteristics to improve LDPC decoding performance. Results show that the developed techniques can reduce LDPC code decoding power consumption by 60% and reduce the decoding failure rate by over two orders of magnitude.