LA-Write: Balancing Endurance of Inter-Layer for Prolonging 3D NAND Flash Memory Lifetime

With vertical stacking, 3D NAND flash memory can achieve continuous capacity growth. However, as the number of stacked layers in a flash block increases, the endurance variation between the stacked layers becomes more and more significant due to process variation, which will seriously affect the lifetime of 3D NAND flash memory. We investigated the endurance variation characteristics between layers and divided the stacked layers into top, middle, and bottom layers according to the endurance characteristics. We found that the endurance of the bottom layer pages is much weaker than that of the other two layers, in response to this endurance variation feature, we proposed a new layer-aware write strategy, called LA-Write. First of all, the write-skip unit in LA-Write will reduce the wear pressure of the pages through write-skip operations. Secondly, LA-Write maintains a layer-aware table, which stores the probability of pages in different layers performing write-skip operation. Setting the probability of the bottom pages to the highest will result in more write-skip operations on the bottom layers, mitigating endurance variations between layers. Experimental results show that LA-Write can increase SSD lifetime by an average of 31%.