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.

The design of survivable storage systems involves inherent trade-offs among properties such as performance, security, and availability. A toolbox of simple and accurate models of these properties allows a designer to make informed decisions. This report focuses on availability modeling. We describe two ways of extending the classic model of availability with a single "correlation parameter" to accommodate correlated failures. We evaluate the efficacy of the models by comparing their results with real measurements. We also show the use of the models as design decision tools: we analyze the effects of availability and correlation on the ordering of data distribution schemes and we investigate the placement of related files. Acknowledgements: This work is partially funded by DARPA/ITO's Organically Assured and Survivable Information Systems (OASIS) Program (Air Force contract number F30602-99-2-0539-AFRL). 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 Page Form Approved OMB No. 0704-0188 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.