Dynamic Hierarchical Caching in Large-Scale Distributed File Systems
Report ID: TR-353-91Author: Alonso, Rafael / Blaze, Matt
Date: 1991-10-00
Pages: 10
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Abstract:
Most Distributed File Systems (DFSs) are based on a flat client-server model in which each client interacts directly with the file server for all file operations. While this model works well for relatively small systems in which the file server has adequate capacity for all its clients, it does not scale to large numbers of clients or systems in which the clients are connected to the server through low-bandwidth links. Server traffic can be reduced substantially if clients keep even a modest-sized cache of previously read files. Intuitively, the benefits of caching can be increased by organizing clients into a hierarchy, in which only a small number of machines communicate directly with the file server, providing intermediate caching services to machines below them in the hierarchy. While this potentially reduces server traffic for widely shared files, it can introduce a significant delay for clients low in the hierarchy for access to files with a low degree of sharing. This paper describes a simple method for constructing dynamic hierarchies on a file-by-file basis. The results of a trace-driven simulation of a dynamic hierarchical filesystem are presented, yielding a reduction in server traffic of a factor of more than two for shared files compared with a flat scheme and without a large increase in client access time. An algorithm to maintain cache consistency with low overhead by detecting missed cache invalidation messages is given.