Download A Cooperative Internet Backup Scheme

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
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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
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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
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•
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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.