The Effect of Skewed Data Access on Buffer Hits and Data Contention an a Data Sharing Environment

In this paper we examine the effect of skewed access on the buffer hit ratio in a multi-system data sharing environment, where each computing node has access to shared data on disks, and has a local buffer of recently accessed granules. In the literature, the effect of skewness in data access on increased data contention has been examined, since with skew most accesses go to few data items. For the same reason, skewness can also increase the buffer hit probability, alleviating the effect on data contention. We examine the resultant effect on the transaction response time, which depends not only on the various system parameters but also on the Concurrency Control (CC) protocol. Furthermore, the CC protocol can give rise to rerun transactions that have different buffer hit probabilities. In a multi-system environment, when a data block gets updated by a system, copies of that block in other system’s local buffers are invalidated. We develop a comprehensive analytical buffer model that captures all these effects and integrate it with a CC model to estimate the overall transaction response time. The model is validated through simulations. We find that higher skew does not necessarily lead to worse performance, and that with skewed access optimistic CC is more robust than pessimistic CC. Examining the buffer hit probability as a function of the buffer size, we find that the effectiveness of additional buffer allocation can be broken down into multiple regions that depend on the degree of skewness. Permission to copq without fee all or part of this material ih granted provided that the copich nre not made or clistrihutcd for direct commercial advantage. the VLDB copyright notice and the title of the publication and its date nppcar. and notice ia gi\cn that copying is by permission of the Vu) Large Data Raw Endowment. To copy otherwise. or to republish. rcquirca :I fee and/or special permission from the Endowment. Proceedings of the 16th VLDB Conferen,ce Brisbane, Australia 1990