ULC: a file block placement and replacement protocol to effectively exploit hierarchical locality in multi-level buffer caches

Abstract

In a large client/server cluster system, file blocks are cached in a multilevel storage hierarchy. Existing file block placement and replacement are either conducted on each level of the hierarchy independently, or by applying an LRU policy on more than one levels. One major limitation of these schemes is that hierarchical locality of file blocks with nonuniform strengths is ignored, resulting in many unnecessary block misses, or additional communication overhead. To address this issue, we propose a client-directed, coordinated file block placement and replacement protocol, where the nonuniform strengths of locality are dynamically identified on the client level to direct servers on placing or replacing file blocks accordingly on different levels of the buffer caches. In other words, the caching layout of the blocks in the hierarchy dynamically matches the locality of block accesses. The effectiveness of our proposed protocol comes from achieving the following three goals: (1) The multilevel cache retains the same hit rate as that of a single level cache whose size equals to the aggregate size of multilevel caches. (2) The nonuniform locality strengths of blocks are fully exploited and ranked to fit into the physical multilevel caches. (3) The communication overheads between caches are also reduced.