DECC: Differential ECC for Read Performance Optimization on High-Density NAND Flash Memory

3D NAND flash memory with advanced multi-level-cell technology has been widely adopted due to its high density, but with significantly degraded reliability. To solve the reliability issue, flash memory often adopts the low-density parity-check code (LDPC) as error correction code (ECC) to encode data and provide fault tolerance. For LDPC with a low code rate, it can provide a strong correction capability, but with a high energy cost. To avoid the cost, LDPC with a higher code rate is always adopted. When the accessed data is not successfully decoded, LDPC will rely on read retry operations to improve the error correction capability. However, the read retry operation will induce degraded read performance. In this work, a differential ECC (DECC) method is proposed to improve the read performance. The basic idea of DECC is to adopt LDPC with different code rates for data with different access characteristics. Specifically, when data is hot read and retried due to reliability, LDPC with a low code rate will be adopted to optimize performance. With this approach, the cost from LDPC with a low code rate is minimized and the performance is optimized. Through careful design and real-world workloads evaluation on a 3D triple-level-cell (TLC) NAND flash memory, DECC achieves encouraging read performance optimization.