In this paper, we report on the experiences in designing a portable parallel file system for clusters. The file system offers to the applications an interface compliant with MPI-IO, the I/O interface of the MPI-2 standard. The file system implementation relies upon MPI for internal coordination and communication. This guarantees high performance and portability over a wide range of hardware and software cluster platforms. The internal architecture of the file system has been designed to allow rapid prototyping and experimentation of novel strategies for managing parallel I/O in a cluster environment. The discussion of the file system design and early implementation is completed with basic performance measures confirming the potential of the approach.