Adapting Layer RBERs Variations of 3D Flash Memories via Multi-granularity Progressive LDPC Reading

Existing studies have uncovered that there exist significant Raw Bit Error Rates (RBERs) variations among different layers of 3D flash memories due to manufacture process variation. These RBER variations would cause significantly diversed read latencies when reading data with traditional Low-Density Parity-Check (LDPC) codes designed for planar flash memories, which induces sub-optimal read performance of flash-based Solid-State Drives (SSDs). To investigate the latency diversity, this paper first performs a preliminary experiment and observes that LDPC read levels proportional to latencies increase in diverse speeds along with data retention. Then, by exploiting the observation results, a Multi-Granularity LDPC (MG-LDPC) read method is proposed to adapt level increase speed for each layer. Five LDPC engines with varied increase granularity are designed to adapt RBER speed requirements. Finally, two implementations for MG-LDPC are applied to assign LDPC engines for each flash layer in a fixed way or dynamically according to prior read levels. Experimental results show that the proposed two implementations can reduce SSD read response time by 21% and 47% on average, respectively.