Data archiving in 1x-nm NAND flash memories: Enabling long-term storage using rank modulation and scrubbing

The challenge of using inexpensive and high-density NAND flash for archival storage was posed recently for reducing data center costs. However, such flash memory is becoming more susceptible to noise, and its reliability issues has become the major concern for its adoption by long-term storage systems. This paper studies the system-level reliability of archival storage that uses 1x-nm NAND flash memory. We analyze retention error behavior, and show that 1x-nm MLC and TLC flash do not immediately qualify for long-term storage. We then implement the rank modulation (RM) scheme and memory scrubbing (MS) for retention period (RP) enhancement. The RM scheme provides a new data representation using the relative order of cell voltages, which provides higher reliability against uniform asymmetric threshold voltage shift due to charge leakage. Results show that the new representation reduces raw bit error rate (RBER) by 45% on average, and using RM and MS together provides up to 196, 171, 146 and 121 years of RPs for blocks with 0, 25, 50 and 75 program/erase cycles, respectively.