Online approaches to physical design tuning have received considerable attention in the recent literature, with a focus on the problem of online index selection. However, it is difficult to draw conclusions on the relative merits of the proposed techniques, as they have been evaluated in isolation using different methodologies. In this paper, we make two concrete contributions to address this issue. First, we propose a benchmark for evaluating the performance of an online tuning algorithm in a principled fashion. Second, using the benchmark, we present a comparison of two representative online tuning algorithms that are implemented in the same database system. The results provide interesting insights on the behavior of these algorithms and validate the usefulness of the proposed benchmark.