Single System Image is a desirable property for all services provided by a cluster environment. Coherently with this principle, cluster file systems should make data distribution transparent to the application, giving rise to a heavy communication workload induced by I/O activity. Hence, I/O performance greatly depends on communication performance and on how the storage and network subsystems interact at all levels. In this paper, we present a detailed performance analysis of the storage and network subsystems of a properly configured cluster node. In particular, we consider the case of an Ultra2 SCSI controller with multiple attached disks sharing the I/O bus with a Gigabit LAN adapter. The analysis helps to understand how the several hardware and software components interact, which are the potential bottlenecks, and how these bottlenecks affect the overall performance.