Recovery of commodity multi-site email clusters

Internet service providers need ways to improve the dependability of their email services. Existing single-site and multi-site, synchronously replicated message stores have low availability. Recovery of multi-site, asynchronously replicated stores is slow and inconsistent. This paper's two-dimensional Markov analysis shows that the availability of an email service is fundamentally limited by site mean-time-to-failure (MTTF), not by the message store MTTF. Replication of the message store at two and three sites can improve the service MTTF by two and four orders of magnitude respectively. One order of magnitude improvement in the storage mean-time-to-repair for two- and three-site replication improves the service MTTF by one and two orders of magnitude respectively. Improvements in the MTTF of current storage technology will not significantly improve email service availability.