In this paper, we propose a novel Single I/O Space architecture for achieving a Single System Image (SSI) at the I/O subsystem level. This is very much desired in a scalable cluster computing environment using commodity components. Our design achieves a single address space for all blocks of data in the cluster, which can tolerate all single disk failures. While traditional approaches focused on at user-level or at distributed file subsystem level, we separate the I/O subsystem of a cluster into the file system and a set of distributed Virtual Device Drivers (VDDs). All SSI services are provided by the VDDs with unmodified file systems. Compared to previous approaches, our approach has higher transparency, better performance, lower implementation cost, higher availability, and application compatibility for I/O intensive cluster computing.