GuardedErase: Extending SSD Lifetimes by Protecting Weak Wordlines

3D NAND flash memory enables the continuous growth in the flash capacity by vertically stacking wordlines (WLs). However, as the number of WLs in a flash block increases, 3D NAND flash memory exhibits strong process variability among different WLs, which makes it difficult for an SSD to fully utilize the maximum endurance of flash blocks, thus reducing the SSD lifetime. In this paper, we propose a new system-level block erase scheme, called GuardedErase, for extending the lifetime of a 3D flash block. The key feature of GuardedErase is that when a block is erased, a WL of the block can be selectively erased by one of two erase modes, the low-stress erase mode or the normal erase mode. When a WL is erased by the low-stress erase mode, the lifetime of the WL can be significantly extended although it may not store data. By supporting two erase modes at the WL level, GuardedErase enables an FTL to exploit the new endurance-capacity trade-off relationship at the SSD level. We have implemented the GuardedErase-aware FTL, called longFTL, which extends the SSD lifetime with a negligible impact on the overall I/O performance. Experimental results using various workloads show that longFTL can improve the SSD lifetime on average by 21% over an existing FTL with little degradation on the SSD performance.