Striping data across multiple nodes has been recognized as an effective technique for delivering high-bandwidth I/O to applications running on clusters. However the technique is vulnerable to disk failure. We present an I/O architecture for clusters called reliable array of autonomous controllers (RAAC) that builds on the technique of RAID style data redundancy. The RAAC architecture uses a two-tier layout that enables the system to scale in terms of storage capacity and transfer bandwidth while avoiding the synchronization overhead incurred in a distributed RAID system. We describe our implementation of RAAC in PVFS, and compare the performance of parity-based redundancy in RAAC and in a conventional distributed RAID architecture.