Error Diluting: Exploiting 3-D nand Flash Process Variation for Efficient Read on LDPC-Based SSDs

3-D NAND flash has become the mainstream in modern SSD designs because it offers superior bit storage density. However, while enjoying the large capacity, 3-D NAND flash is highly prone to bit errors due to its cylindrical cell structure. Modern SSDs employ the low-density parity-check (LDPC) error-correcting code to manage bit errors in 3-D NAND flash. Strong LDPC error correction is subject to a high time overhead, because it may require many sensing levels on read to obtain sufficiently confident bit input information. By exploiting the bit-error rate variation among vertical layers of 3-D NAND flash, we propose diluting bit errors of cells at error-prone, lower layers by mixing them with bit data of cells from reliable, upper layers. Cells at reliable layers provide highly confident bit input information that helps reduce the number of sensing levels on cell at error-prone layers. Our experimental results showed that the proposed approach improved the read throughput by 29% and reduced the read latency by 43% compared with a conventional multichip SSD design.