MCBGC: A Multi-Threshold Copyback-based Garbage Collection Scheme for 3D NAND Flash Memory

Garbage collection (GC) is critical to improving 3D NAND flash memory space utilization. However, GC is very time-consuming for migrating valid data for error correction, which leads to a sharp decline in system performance. Copyback is an advanced command that can be used to accelerate data migration in GC. However, existing copyback-based GC scheme can not guarantee data reliability for various types of flash pages and cause a large decoding latency in the error correction process. In this paper, we first evaluate the copyback error characteristics that measure the quantitative relationships between the number of copybacks and the raw bit error rate (RBER) of upper, middle, and lower pages of triple level cell NAND flash through practical testing, respectively. Then we propose a multi-threshold copyback-based GC scheme (MCBGC). Under the specific RBER limit, the copyback thresholds of upper, middle, and lower pages are determined respectively based on the copyback error characteristics. Experimental results show that compared with the existing copyback-based GC scheme, the write latency of the proposed scheme can be improved by up to 17.7%.