A Swap System On NAND Flash With Compressed Cache

Some smart phones and laptop computers now exploit swap system with their second storage media as a cost effective solution to extend limited memory space. The rapid evolving flash memory technology starts to replace the magnetic disks of these computers by NAND flash due to its advantageous characteristics such as energy efficiency, access speed and mechanical shock resistance. Although NAND Flash is currently considered as the second mass-storage media, we can imagine that the swap system running over NAND flash will show up in the future.However, NAND suffers limited lifespan compared with magnetic disks. Blocks of NAND Flash will be destroyed if certain blocks are erased too many times. As a result, our research focuses on how to reduce writing operations on NAND Flash while swapping in and out.This paper presents a new swap system model, SNSS (Smart NAND Swap System). And a compressed cache in swap system is introduced, as well as two-level replacement algorithm and adaptive compression, which are intend to reduce access to NAND.A compressed cache is used as middle layer between main memory and NAND swap area. The cache and the NAND share the same address space of page slots. Each swapped page will be compressed first and then allocated with some page slot, in cache or NAND or maybe both.The two-level replacement algorithm introduced by SNSS is designed to be that pages in RAM are replaced following LRU like tradition, and pages in compressed cache are replaced with a special algorithm which performs "Clean page First, Then New, Dirty Last" while rewriting on NAND costs too much and creating a new page costs less.Further more, adaptive compression is introduced in SNSS, which is used to avoid compression under low compression rate.By simulating the SNSS model, we get the conclusion that SNSS can reduce flash write operations by 50% on average compared with the traditional swap system. As a result, NAND will keep healthy twice longer.