Myriad: Cost-Effective Disaster Tolerance

This paper proposes a new approach for achieving disaster tolerance in large, geographically-distributed storage systems. The system, called Myriad, can achieve the same level of disaster tolerance as a typical single mirrored solution, but uses considerably fewer physical resources, by employing cross-site checksums (via erasure codes) instead of direct replication. The key technical contribution of the paper is a protocol permitting cross-site checksums to be updated in such a way that data recovery is always possible. Another important contribution is the specification of a protocol for recovering from disasters, explicitly verifying the claim of disaster tolerance. Further, it is shown by direct calculation and analytical modeling that Myriad compares favorably with mirroring in terms of both total cost of ownership and reliability.