A fault-scalable service can be configured to tolerate increasing numbers of faults without significant decreases in performance. The Query/Update (Q/U) protocol is a new tool that enables construction of fault-scalable Byzantine faulttolerant services. The optimistic quorum-based nature of the Q/U protocol allows it to provide better throughput and fault-scalability than replicated state machines using agreement-based protocols. A prototype service built using the Q/U protocol outperforms the same service built using a popular replicated state machine implementation at all system sizes in experiments that permit an optimistic execution. Moreover, the performance of the Q/U protocol decreases by only 36% as the number of Byzantine faults tolerated increases from one to five, whereas the performance of the replicated state machine decreases by 83%.

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Availability and confidentiality of information are primary goals of many information systems. Most systems enhance availability by providing full replication, The PASIS architecture flexibly and efficiently combines proven technologies for constructing information storage systems whose availability, confidentiality, and integrity policies can survive component failures and malicious attacks.

We thank the members and companies of the PDL Consortium (including EMC, HP, Hitachi, IBM, Intel, Network Appliance, Panasas, Seagate, Sun and Veritas) for their interest, insights and support. We also thank Microsoft Corporation for sharing their availability measurements of the desktops in their campus. Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

Large scale storage systems require multi-disk fault tolerant erasure codes. Replication and RAID extensions that protect against two-and three-disk failures offer a stark tradeoff between how much data must be stored, and how much data must be read to recover a failed disk. Flat XOR-codes-erasure codes in which parity disks are calculated as the XOR of some subset of data disks-offer a tradeoff between these extremes. In this paper, we describe constructions of two novel flat XOR-code, Stepped Combination and HD-Combination codes. We describe an algorithm for flat XOR-codes that enumerates recovery equations, i.e., sets of disks that can recover a failed disk. We also describe two algorithms for flat XOR-codes that generate recovery schedules, i.e., sets of recovery equations that can be used in concert to achieve efficient recovery. Finally, we analyze the key storage properties of many flat XOR-codes and of MDS codes such as replication and RAID 6 to show the cost-benefit tradeoff gap that flat XOR-codes can fill.