Accelerating garbage collection for 3D MLC flash memory with SLC blocks
Shuai Li, Wei Tong, Jingning Liu, Bing Wu, Yazhi Feng · 2019
3D MLC NAND Flash is more appreciated for its massive capacity and significant performance. It's common to configure a portion of flash blocks to SLC-mode to further shorten the requests latency at a small cost of capacity. However, the limited SLC-mode blocks provoke GC (Garbage Collection) procedures more frequently and the GC penalty is heavier for 3D Flash than that for 2D Flash. As the block of 3D Flash consists of much more pages, the increment in the block size prolongs the erase operation latency and increases the number of migrated pages during GC. Existing works focus on reducing the number of migrated pages with sub-block GC strategies, which deeply depends on the distribution of valid pages across the sub-blocks in the victim block. In this paper, we propose a set of schemes called DCD which consists of Dual-mode Handler, Compensated GC and Dynamic Data Distribution. The key idea is to exploit the shorter operation delays of SLC-mode to accelerate the valid pages' migrations during GC, which doesn't rely on the distribution of valid pages inside the victim block. Experimental results show that compared with state-of-the-art designs, DCD shortens the average read response time by 19.8% and 37.3% for block-level and page-level FTL, respectively.