Support of Single System Image (SSI) services is the main approach that enables better utilization of PC/workstation clusters. Some SSI services can be easily built with the support of other low-level, elementary, SSI services. In this paper, we describe Single I/O Space architecture for achieving a SSI at the I/O subsystem level. Furthermore, we demonstrate how the Single I/O Space can facilitate the development of other key SSI services. Typical SSI services, which can benefit from the Single I/O Space, include single file hierarchy, single memory space, checkpointing systems and single process space with process migration facilities. Benchmark performance results show that our design achieves both performance and storage size scalabilities that are essential to building I/O-intensive clusters.