RACS: A Case for Cloud Storage Diversity

1. RACS: A Case for Cloud Storage Diversity Paper by Hussam Abu-Libdeh Cornell University Lonnie Princehouse Cornell University Hakim Weatherspoon Cornell University (1st ACM Symposium on Cloud Computing, SoCC 2010, Indianapolis, Indiana, USA, June 10-11, 2010. ACM 2010) Presented by Chayutra Pailom CS Department, UTSA

2. What is this topic about? • Motivation: Diversify cloud storage providers • Key issues: The risk on being service on solely particular cloud storage provider • Solutions: Strip data across vendors • Comparisons: Overhead expense and vendor mobility • Possible improvement: Avoid vendor lock-in, reduce the cost of switching providers and better tolerate provider outages or failures. • Number of References: 39 references

3. Agenda Terminology Background Concepts Evaluation Conclusion

4. Related Terminology • RAID (Redundant Array of Independent Disks) RAID 0 (Striped) Data is spread across all drive volumes. Drive volumes must be the same size. If one drive fails, all data is lost RAID 5 Data is spread across all drives, as is error-correcting parity information. If one drive fails, data can be reconstructed from the remaining drives. RAID 1 (Mirrored) Data is copied to two drives simultaneously. Mirrored drive volumes must be the same size. If one drive fails, data can still be accessed from the mirrored drive. Picture: http://www.microcenter.com/random_access/newsletters/06_newsletters/0306/in_the_lab.html

5. Related Terminology • RAID 5 (Striping With Parity) http://www.vectortech.co.th/technic_002.php http://www.dataclinic.com/hard-disk-raid-recovery.htm

6. Related Terminology • RAID (Redundant Array of Independent Disks) Take 1 minute to take this visualization for better understanding http://www.acnc.com/raidedu/0

7. Related Terminology • Vendor lock-in • A situation in which a cloud customer is “stuck” to his/her current cloud vendor due to the complexity in switching to another cloud vendor. • you could “unlock” your business from your current cloud vendor – but that not without “sacrifices,” in term of time, money and effort • RACS • Redundant Array of Cloud Storage • Use RAID 5 as a prototype • Disk = Cloud Storage http://www.cloudbusinessreview.com/2011/05/03/how-to-avoid-cloud-vendor-lock-in.html

8. Related Terminology • Error Correction Code • Use Reed-Solomon error correction code. • Oversampling a polynomial constructed from the data. • The polynomial is evaluated at several points, and these values are sent or recorded. • Sampling the polynomial more often than is necessary makes the polynomial over-determined. • As long as it receives "many" of the points correctly, the receiver can recover the original polynomial even in the presence of a "few" bad points. • Used in many different kinds of commercial applications, for example in CDs, DVDs and Blu-ray Discs. Credit: http://simple.wikipedia.org/wiki/Reed-Solomon_error_correction

9. Related Terminology • Erasure coding (human understandable version???) • The key idea behind a Reed-Solomon code is that the data encoded is first visualized as a polynomial. • The code relies on a theorem from algebra that states that any k distinct points uniquely determine a polynomial of degree at most k-1. • The sender determines a degree k − 1 polynomial, over a finite field, that represents the k data points. The polynomial is then "encoded" by its evaluation at various points, and these values are what is actually sent. • During transmission, some of these values may become corrupted. Therefore, more than k points are actually sent. • As long as sufficient values are received correctly, the receiver can deduce what the original polynomial was, and decode the original data. Credit: http://simple.wikipedia.org/wiki/Reed-Solomon_error_correction

10. The authors mentioned that cloud storage is more reliable than hard drives so they leave the readers of this paper assumed less failure of the data and not mentioned that much!

11. RACS: A Case for Cloud Storage Diversity

12. Agenda Terminology Background Concepts Evaluation Conclusion

13. Background • The Cloud Storage Market • Cloud storage providers expose simple interfaces to developers. Amazon S3's data model provides at namespaces (buckets) into which named objects can be uploaded for later retrieval. • Other storage services can be mounted as network file systems. There is no widely agreed-upon standard interface, but S3's REST API has been adopted by smaller providers and by the open-source Eucalyptus server software.

14. Background • The Cloud Storage Market • These interfaces differ, but are similar enough to be considered interchangeable. Storage providers are forced to compete on price rather than by offering unique services. • Cloud storage is a highly competitive market. These are simplified pricing schemes for the top two cloud storage providers; Amazon S3 and Rackspace.

15. Introduction Pricing scheme of different cloud storage providers

16. Background • Vendor lock-in • Expensive to switch. • Subject to possibility of data loss. • For business, it is very big issue; go cloud or not. • Ways to fix, e.g. Economical approach spread the data across multiples providers. • But high storage and bandwidth cost. • Use this paper’s approach!

17. Background • Why should we diversify… • Outages and operational failures. • Economic failures. • By striping data across providers and adding appropriate redundancy, clients can tolerate outages and operational failures, as well as adapt to changes in the economic matters. • Avoid vendor lock-in!

18. Background • Incentives for Redundant Striping • RACS uses Reed-Solomon error correcting codes to tolerate failures without data loss. Starting with m equal-size disks of original data, we fill k additional disks with redundant data. Any combination of m disks (data or redundant) is sufficient to reconstruct the original data. We write (m, n) to indicate that there are n = m + k total disks, any m of which are sufficient to reconstruct all original data Credit: http://pubs.0xff.co/posters/racs.pdf

19. Background • Incentives for Redundant Striping (cont.) • The choice of parameters m and n is a trade-off: Overhead for data storage and write operations is increased by the ratio n : m. Interestingly, read operations are not significantly more expensive, since only m disks must be read under normal operating condition • RAID-5 uses a similar strategy to tolerate up to one failure in an array of hard disks.

20. Agenda Terminology Background Concepts Evaluation Conclusion

21. Redundant Array of Cloud Storage RACS single-proxy architecture

22. Redundant Array of Cloud Storage RACS single-proxy architecture

23. Redundant Array of Cloud Storage • The goal of RACS is slightly different than RAID-5. Cloud storage is assumed to be much more reliable than hard disks, so data loss prevention is a much less compelling reason to use error correcting codes. • RACS lowers the cost of switching providers, e.g., as a result of economic failure. • Only 1/m of all data needs to be moved to leave a vendor. • By reducing the impact of vendor lock-in, RACS increases the leverage of customers when negotiating contracts with cloud providers. • RACS is implemented as an HTTP proxy with the same interface as Amazon S3.

24. Redundant Array of Cloud Storage Multiple RACS proxies coordinate their actions using ZooKeeper

25. Agenda Terminology Background Concepts Evaluation Conclusion

26. Evaluation • Internet Archives Trace • Trace represents 18 months of activity on the Internet Archive’s FTP site. • What are the estimated costs associated with storing library type content such as the Library of Congress and the Internet Archive on the cloud? • What is the cost of changing storage providers for large organizations? What is the added cost of a DuraCloud-like (replicate data across multiples cloud providers) • What is the cost of avoiding vendor lock-in using RACS? And what is the added cost overhead of using RACS (depends on the coding configuration = additional data to be written)?

27. Evaluation Inbound and outbound data transfers in the Internet Archive trace for Individual IA data transfers Read/write requests in the Internet Archive trace for Individual IA read/write requests

28. Evaluation • Cost of Moving to The Cloud Estimated monthly and cumulative costs of hosting on the cloud with different storage providers

29. Evaluation • Cost of Switching Vendors The cost of switching the Internet Archive’s storage provider for various configurations

30. Agenda Terminology Background Concepts Evaluation Conclusion

31. Conclusion • The commoditization of cloud services has brought with it the characteristics of an economy, good and bad. • RACS: A simple application of technology to change the structure of a market. • Erasure coding is applied to different types of failure (economic) than in the storage systems. • Microbenchmarks and larger experiments are simulated for the real-world traces. • RACS enables cloud storage customers to explore trade-offs between overhead and mobility.

32. Sample References [16] J. Bloemer, M. Kalfane, M. Karpinski, R. Karp, M. Luby, and D. Zuckerman. An XOR-based erasure-resilient coding scheme. Technical Report TR-95-048, The International Computer Science Institute, Berkeley, CA, 1995. [30] D. Patterson, G. Gibson, and R. Katz. The case for RAID: Redundant arrays of inexpensive disks. In Proc. of ACM SIGMOD Conf., pages 106–113, May 1988. [38] M. Vrable, S. Savage, and G. M. Voelker. Cumulus: Filesystem backup to the cloud. Trans. Storage, 5(4):1–28, 2009.

33. Questions?