Database buffer model for the data sharing environment

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Abstract
A simple analytic buffer model for coupled systems sharing a common database is developed. When a data block gets updated by a system, copies of that block in other systems' buffers need to be invalidated. This cross-invalidation phenomenon makes the estimation of the buffer hit ratio interesting. The concurrency control (CC) scheme can also complicate the situation. Under the optimistic CC protocol, conflicting transactions can get aborted and need to be rerun. Rerun transactions tend to exhibit a higher hit ratio than first-run transactions since reference blocks, except those that were invalidated, are likely to continue to reside in the buffer. On the other hand, the buffer hit ratio affects not only the number of IOs but also the abort probability. The authors develop a buffer model to capture these effects and integrate it with a CC model to estimate the overall transaction response time Since the invalidation rate increases with buffer size, the advantage of a larger buffer size in the data-sharing environment becomes an issue. The model is then used to study this issue and estimates the maximum buffer that can be usefully employed in each system. The analysis is validated through simulations.

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