Multi-Site Declustering Strategies for Very High Database Service Availability

The thesis introduces the concept of multi-site declustering strategies with self repair for databases demanding very high service availability. Existing work on declustering strategies are centered around providing high performance and reliability inside a small geographical area (site). Applications demanding robustness against site failures like fire and power outages, can not use these methods. Such applications will often both replicate information inside one site and then replicate the site on another site and thus resulting in unnecessary high redundancy cost. Multi-site declustering provides robustness against site failures with only two replicas of data without compromising the performance and reliability. Self repair is proposed for reducing the probability of double-failures causing data loss and reducing the need for rapid replacement of failed hardware. A prerequisite for multi-site declustering with self repair is fast, long-distance, communication networks like ATM. The thesis shows how existing declustering strategies like Mirrored, Interleaved, Chained, and HypRa declustering can be used as multi-site declustering strategies. In addition a new strategy called Q-rot declustering is proposed. Compared with the others it gives larger flexibility with respect to repair strategy, number of sites, and usage pattern. To evaluate availability of systems using the methods a general evaluation model has been developed. Multi-site Chained declustering provides the best availability of the methods evaluated. Q-rot declustering has comparable availability but is significantly more flexible. The evaluation model provides insight and can be used to understand the declustering problem better and to develop new and improved multi-site declustering strategies. The model can also be used as a configuration tool by organizations wanting to deploy one of the declustering strategies.