FlexFS: A Flexible Flash File System for MLC NAND Flash Memory

The multi-level cell (MLC) NAND flash memory technology enables multiple bits of information to be stored on a single cell, thus making it possible to increase the density of the memory without increasing the die size. For most MLC flash memories, each cell can be programmed as a single-level cell or a multi-level cell during runtime. Therefore, it has a potential to achieve both the high performance of SLC flash memory and the high capacity of MLC flash memory. In this paper, we present a flexible flash file system, called FlexFS, which takes advantage of the dynamic reconfiguration facility of MLC flash memory. FlexFS divides the flash memory medium into SLC and MLC regions, and dynamically changes the size of each region to meet the changing requirements of applications. We exploit patterns of storage usage to minimize the overhead of reorganizing two different regions. We also propose a novel wear management scheme which mitigates the effect of the extra writes required by FlexFS on the lifetime of flash memory. Our implementation of FlexFS in the Linux 2.6 kernel shows that it can achieve a performance comparable to SLC flash memory while keeping the capacity of MLC flash memory for both simulated and real mobile workloads.