R&D: Page Type-Aware Data Migration Technique for Read Disturb Management of NAND Flash Memory

IEEE Transactions on Very Large Scale Integration (VLSI) Systems has published an article written by Sangwoo Han, Minjung Cho, Gi Lee, and Eui-Young Chung, School of Electrical and Electronic Engineering, Yonsei University, Seoul, South Korea.

Abstract: “Read disturb is a phenomenon in which the cells of all unselected wordlines (WLs) within a block are soft programed while reading a selected WL, which causes errors. To solve the problem of read disturb, read reclaim (RR) operations, which migrate deteriorated block data to a new free block, are executed when the read count of the deteriorated block exceeds a preset threshold. RR requires additional operations, such as error correction, reprogramming, block erasure, and occupying I/O. So as the number of RR operations increases, the performance and reliability of the memory degrade. We observe that read disturb affects pages within a block differently according to page types. With this perspective, we propose Page Type-aware Data Migration technique (PTDM) technique, which migrates pages within blocks according to the vulnerability to read disturb. Shifting RR from the conventional block level to the page level improves the efficiency of the memory by reducing data migration. Also, we propose distributing read-hot data into several blocks during migration to reduce read disturb impact. Our experimental results show that the proposed method can reduce the amount of RR data migration and the overall erasure counts by 39.53% and 21.69%, respectively.“