Design of cluster file system is very important for building a general-purpose cluster with commodity components. To provide scalable high I/O performance needed in the scientific computing, engineering computing, Internet/Intranet services, I/O intensive commercial database and multimedia application, we designed a high-performance cluster file system named COSMOS. This paper presents the basic characteristics of the cluster file system and its performance analysis. The performance of the COSMOS file system is evaluated on the platform of Dawning/2000 superserver, using standard I/O performance benchmarks. Our tests show satisfactory bandwidth and overall performance of COSMOS, and the system scales well. Based on implementation experience and the raw data, we discuss the bottleneck of the existing system and propose methods to improve the system. Keywords cluster file system, performance analysis, single-system image, scalability