Design and Applications of Dependable Nonvolatile Memory Systems

Abstract

This chapter discusses how to build dependable nonvolatile memory systems that range from the SD card to high-performance enterprise storage. The nonvolatile memory systems are mainly composed of NAND flash memories because of their high bit density. However, the reliability of the NAND flash memory is degrading along with the memory-cell scaling. Therefore, the adoption of the highly reliable techniques is becoming increasingly important. On the other hand, storage-class memories (SCMs) are attracting much attention because of the higher performance than NAND flash memories. Since the cost is higher than the NAND flash memories, NAND flash/SCM hybrid configuration is developed. Therefore, some of the techniques introduced in this chapter are for the hybrid storage where SCMs are exploited as nonvolatile buffer. Dynamic codeword transition ECC and error-prediction (EP-) low-density parity-check (LDPC) schemes are the techniques related to the error-correcting codes (ECCs). Page-RAID, reverse mirroring (RM), and shift mirroring (SM) are described as redundant arrays of independent disks (RAID). Moreover, data preprocessing techniques such as asymmetric coding and stripe pattern elimination algorithm (SPEA) are introduced. Error recovery (ER) and error masking (EM) schemes are shown as the techniques which cannot be fit into the above classifications. To design dependable nonvolatile systems, techniques should be selectively applied from each layer (ECC, RAID, data preprocessing, and others) to satisfy the cost, performance, and reliability requirements of the application. Therefore, the storage overhead (cost), performance, and the acceptable bit error rate improvement (reliability) are compared among techniques in this chapter.