Distributed file systems have become popular in recent years. However, they still lack functions for doing fast arbitrary data insertion and truncation. To solve the problem, we present Wofs, an object-based distributed network file system which supports fast arbitrary data insertion and truncation. Wofs splits a file into many small objects, stores these objects in remote file servers, and uses a special B+tree to manage the metadata of these objects. Besides, Wofs uses the object-range locking policy to avoid data incoherence and improve performance.