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Advanced Topics in Storage Systems A Cooperative Internet Backup Scheme [1] Leonid Bilevich [1] M. Lillibridge, S. Elnikety, A. Birrell, M. Burrows, and M. Isard, A cooperative Internet backup scheme, Proc. USENIX Annual Technical Conference, San Antonio, TX, June 2003. Outline • • • • • Problem Main findings Technical description of the results Possible extension Conclusion Problem Addressed Need: Backup Constraint: Small budget Solution: Peer-to-peer network Main Findings The suggested system is: – Cheap – Reliable – Secure Main Features of the System • Pros: – Cheapness (the hardware is available) – Diversification (different sites and different operation systems) • Cons: – Unreliability of one computer – Non-cooperative environment ---------------------------------------------------• Solution: – Redundancy Reliability – Security mechanisms The simplified scheme (cooperation) • Each computer runs a backup program • Partner computers agree on amount of storage and uptime Main Steps • Backing up data • Restoring data Reed-Solomon Erasure-Correcting Codes Backing up data Restoring data Reliability and Overhead Example • Reliability of computer=90% • Reliability of the system: Security • Confidentiality • Integrity • Free-rider attacks Confidentiality • Encrypt the data before sending Integrity • Authenticate the partner with secret key • Use a keyed cryptographic hash as checksum Free-rider attacks Agreement violations • Problem: Using backup service without contributing backing service • Solution: “challenge” – testing if the data is stored Exploiting the grace period • Problem: The grace period when the computer can be down is exploited • Solution: Read prohibition (using low-utility blocks) Bandwidth theft • Problem: Computers are used to broadcast information • Solution: Quota on number of reads and writes per day Performance • The network + remote disk step consumes the largest portion of backup time. Cost • For existing Internet backup system: minimal cost= 7.20 $/GB/month • For the new system: minimal cost= 18.6 US cents/GB/month ------------------------------------------------------------------------------------------------------• Saving: 30 ÷ 100 times Limitations • Limited grace period • Restoration is impossible during the grace period • Long restoration time • Vulnerability to catastrophic viruses ------------------------------------------------------------------------• Solution: hybrid system with central server Other systems Old systems • Eternity Service • Archival Intermemory • Free Net • Free Haven New systems • PAST • OceanStore • Pastiche Possible extension Present work – occasional testing • We merely verify that computers don’t drop data. Another work – rigorous testing • The disk scrubbing technique [2] verifies the integrity of data. -------------------------------------------------------------------------------------------------------------Combined approach • We can incorporate the opportunistic disk scrubbing into the present work, verifying the validity of data and improving the reliability of the backup system. Conclusion Cooperative Internet backup system • Cheapness • Reliability • Security • Possible extension References • • • M. Lillibridge, S. Elnikety, A. Birrell, M. Burrows, and M. Isard, A cooperative Internet backup scheme, Proc. USENIX Annual Technical Conference, San Antonio, TX, June 2003. J. S. Plank, A tutorial on Reed-Solomon coding for fault-tolerance in RAID-like systems, Software: Practice and Experience, 27(9):995–1012, Sep. 1997. Correction in: J. S. Plank and Y. Ding, Technical Report UT-CS-03-504, Univ. Tennessee, 2003. T. J. E. Schwarz, Q. Xin, E. L. Miller, D. D. E. Long, A. Hospodor, and S. Ng, Disk scrubbing in large archival storage systems, Proc. MASCOTS, Volendam, Netherlands, October 2004.