Recent mobile devices adopt various flash storages as a primary storage. File system support for those flash storages is a must for flash device performance and lifespan.I will present a new file system, called F2FS, designed for mobile flash storages. F2FS is designed considering the characteristics of the underlying flash storage which has flash translation layer (FTL). F2FS outperforms EXT4, which is a popular file system for Android phones, in most of benchmarks. I will describe motivation, design, and implementation of the file system, then show performance comparison data with EXT4. Target audiences are those who are interested in file system support for flash storages such as eMMC and SSD. Kernel and file system expertise helps but is not mandatory to listen to this talk.