The storage and access of massive small files are one of the challenges in the design of DFS(Distributed file system). Most of the DFSs, such as GFS and HDFS, are designed for handling massive big files. The performance of DFSs decreases greatly when accessing massive small files without special optimization for small files. This research focuses on the optimizing of the performance of data server in handling massive small files, and presents a Flat Lightweight File System called FlatLFS in which the user data are managed flat in disks. FlatLFS is supposed to substitute the traditional file system when accessing user data for upper DFSs. With the improvement of the performance of small data block processing on data servers by FlatLFS, the performance of the whole DFSs is greatly improved. The effectiveness of FlatLFS is proved with intensive experiments: when the size of data block is 1M, the performance of random read of FlatLFS is 135%, 112% and 122% higher than ext 3 ,ext4 and reiserfs respectively. 