Layer RBER Variation Aware Read Performance Optimization for 3D Flash Memories

3D NAND flash enables the construction of large capacity Solid-State Drives (SSDs) for modern computer systems. While effectively reducing per bit cost, 3D NAND flash exhibits non-negligible process variations and thus RBER (raw bit error rate) difference across layers, which leads to sub-optimal read performance for applications with either small or large I/O requests. In this paper, we propose LRR, Layer RBER variation aware Read optimization schemes, to address the challenge. LRR consists of two schemes - LRR subpage read scheduling (SRS) and LRR fullpage allocation (FPA). SRS groups small read requests from the layers with similar RBERs to reduce the average read latency of subpage sized read requests. FPA distributes the data of a large write to multiple layers, which improves the read latency when reading from layers with large RBERs. Our experimental results show that our proposed scheme LRR reduces 46% read latency on average over the state-of-the-art.