This invention is a system and a method for operating a storage server in a data network using a new architecture. The method of creating the partial block map allows the snapshot writes on a direct mapped file. The method of reading data or writing data to the file created in direct mapping state and later converted to partial mapping state responsive to a copy on first write request by a client allocates new indirect block when needed to store the reference to newly allocated data block. The method of reading data from or writing data to the file in partial mapping state involves checking the mapping bit to find if the indirect block is in direct mapping state.