Performance Analysis of Dynamic Finite Versioning Schemes: Storage Cost vs. Obsolescence

Dynamic finite versioning (DFV) schemes are an effective approach to concurrent transaction and query processing, where a finite number of consistent, but maybe slightly out-of-date, logicalsnapshots of the database can be dynamically derived for query access. In DFV, the storage overhead for keeping additional versions of changed data to support the logical snapshots and the amount of obsolescence faced by queries are two major performance issues. In this paper, we analyze the performance of DFV, with emphasis on the trade-offs between the storage cost and obsolescence We develop analytical models based on a renewalprocess approximation to evaluate the performance of DFV using M t 2 snapshots. Asymptotic closed-form results for high query arrival rates are given for the case of two snapshots. Simulation is used to validate the analytical models and to evaluate the tradeoffs between various strategies for advancing snapshots when M > 2. The results show that 1) the analytical models match closely with simulation; 2) both the storage cost and obsolescence are sensitive to the snapshot-advancing strategies, especially for M > 2 snapshots; and 3) generally speaking, increasing the number of snapshots demonstrates a trade-off between storage overhead and query obsolescence. For cases with skewed access or low update rates, a moderate increase in the number of snapshots beyond two can substantially reduce the obsolescence, while the storage overhead may increase only slightly, or even decrease in some cases. Such a reduction in obsolescence is more significant as the coefficient of variation of the query length distribution becomes larger. Moreover, for very low update rates, a large number of snapshots can be used to reduce the obsolescence to almost zero without increasing the storage overhead.