Multi-level Caching in Distributed File Systems

Abstract

We are investigating the potential for intermediate file servers to address scaling problems in increasingly large distributed file systems. To this end, we have run trace-driven simulations based on data from DEC-SRC and our own data collection to determine the potential of caching-only intermediate servers. The degree of sharing among clients is central to the effectiveness of an intermediate server. This turns out to be quite low in the traces available to us. All told, fewer than 10% of block accesses are to files shared by more than one file system client. Trace-driven simulation shows that even with an infinite cache at the intermediate, cache hit rates are disappointingly low. For client caches as small as 20 MB, we observe hit rates less than 19%. As client cache sizes increase, the hit rate at the intermediate approaches the degree of sharing among all clients. On the other hand, the intermediate does appear to be effective in reducing the peak load presented to upstream file servers.