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Introspective Replica Management in OceanStore

This paper discusses the problems caused by poor replica management in OceanStore and proposes a solution using introspection and prefetching to improve latency, robustness, and load sharing. The paper also explores different methods for prefetching and the role of proxies in the system.

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Introspective Replica Management in OceanStore

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  1. Introspective Replica Management in OceanStore Dennis Geels ROC/OceanStore Winter Retreat 2002

  2. Problem Description ROC/OceanStore Winter Retreat 2002

  3. Replica Data Structure • Heartbeats • Signed (name  version) mapping • Version Metadata • Chain of Metadata blocks for recent versions • Resource Accounting • % of each version stored locally • Access frequency • Payment/contract info (eventually) ROC/OceanStore Winter Retreat 2002

  4. Read Algorithm and Latencies • Open Replica • One Tapestry lookup round trip • Determine version • One round trip to parent replica • Read BTree • Round trip to parent replica per level of BTree, or • Forward request to parent (one large transfer) ROC/OceanStore Winter Retreat 2002

  5. Why Bother? • Problems caused by poor Replica Management: • High access latency • Sensitivity to Network Failure • Poor load sharing ROC/OceanStore Winter Retreat 2002

  6. Solution • Keep Replica near clients • Makes OceanStore behave like a local area file system • Improves latency, robustness, load sharing • Do work ahead of time • Avoids initial wide-area hit ROC/OceanStore Winter Retreat 2002

  7. How to Prefetch? • Algorithm: • Cluster files • Manually / automatically • Detect access to Cluster • Possibly predict use via temporal patterns? • Prefetch Cluster in stages • Initiate read process • More effort expended on most likely files ROC/OceanStore Winter Retreat 2002

  8. Who Prefetches? • Client machines • Access to user activity stream • Limited resources • Proxy machine • Off-node helper with extra storage, better network connectivity ROC/OceanStore Winter Retreat 2002

  9. Where do we find proxies? • Choose machine along search path • Saves work in following Tapestry lookup • Keep proxy on retainer • Quick lookup, simple contracts • Bulletin board • Post requests to third-party server • Any available proxy responds ROC/OceanStore Winter Retreat 2002

  10. Example: Using Search Path • Clients dispatch prefetch requests along normal read path • Can use TTL to place more important replicas nearer Tapestry lookup path ? ? ? ROC/OceanStore Winter Retreat 2002

  11. Example: Proxies on Retainer Tapestry lookup path • Client machines maintain contact with small number of local proxies • Can find resources off lookup path • Direct communication, simple reputation management ? ? ROC/OceanStore Winter Retreat 2002

  12. Example: Using Bulletin Board • Third party accepts requests for various services • Local proxies with available resources receive updates • Very flexible, general infrastructure support for proxies Tapestry lookup path ? ? ? ROC/OceanStore Winter Retreat 2002

  13. Future Work • Smart decisions when traffic dominated by updates • Fewer replicas often better • Large scale testing, simulation ROC/OceanStore Winter Retreat 2002

  14. Conclusion Introspective Replica Management may negate the problems of wide-area storage OceanStore will (hopefully) feel like a local network file system. ROC/OceanStore Winter Retreat 2002

  15. Acknowledgements • My Wife, Cheryl ROC/OceanStore Winter Retreat 2002

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