A cluster of PCs can be seen as a collection of networked low cost disks; such a collection can be operated by proper software so as to provide the abstraction of a single, larger block device. By adding suitable data redundancy, such a disk collection as a whole could act as single, highly fault tolerant, distributed RAID device, providing capacity and reliability along with the convenient price/performance typical of commodity clusters. We report about the design and performance of DRAID, a distributed RAID prototype running on a Gigabit Ethernet cluster of PCs. DRAID offers storage services under a Single I/O Space (SIOS) block device abstraction. The SIOS feature implies that the storage space is accessible by each of the stations in the cluster, rather than throughout one or few end-points, with a potentially higher aggregate I/O bandwidh and better suitability to parallel I/O.