Exploiting process similarity of 3d flash memory for high performance SSDs

3D NAND flash memory exhibits two contrasting process characteristics from its manufacturing process. While process variability between different horizontal layers are well known, little has been systematically investigated about strong process similarity (PS) within the horizontal layer. In this paper, based on an extensive characterization study using real 3D flash chips, we show that 3D NAND flash memory possesses very strong process similarity within a 3D flash block: the word lines (WLs) on the same horizontal layer of the 3D flash block exhibit virtually equivalent reliability characteristics. This strong process similarity, which was not previously utilized, opens simple but effective new optimization opportunities for 3D flash memory. In this paper, we focus on exploiting the process similarity for improving the I/O latency. By carefully reusing various flash operating parameters monitored from accessing the leading WL, the remaining WLs on the same horizontal layer can be quickly accessed, avoiding unnecessary redundant steps for subsequent program and read operations. We also propose a new program sequence, called mixed order scheme (MOS), for 3D NAND flash memory which can further reduce the program latency. We have implemented a PS-aware FTL, called cubeFTL, which takes advantage of the proposed techniques. Our evaluation results show that cubeFTL can improve the IOPS by up to 48% over an existing PS-unaware FTL.