When a file server is used, performance of file I/O can be noticeably degraded relative to local file access because of the delays imposed by the network. Individual systems can use file caches to hold recently accessed file records. Because of the principal of locality, use of a local file cache should reduce the number of remote server accesses that must be made. When a process makes a file access, the request is presented first to the cache of the process`s work station.
If not satisfied there, the request is passed either to the local disk if the file is stored is there, or to a file server where the file is stored. At the server, the server`s cache is first interrogated and if there is a miss then the servers disk is accessed. The dual caching approach is used to reduce communications traffic and disk I/O.
When caches always contain exact copies of remote data, then the caches are consistent. It is possible for caches to become inconsistent when the remote data are changed and the corresponding obsolete local cache copies are not discarded. This can happen if one client modifies a file that is also cached by other clients. The difficulty is actually at two levels. If a client adopts a policy of immediate writing any changes to a file back to the server, then any other client with a cache copy of the relevant portion of the file back will have obsolete data. The problem is made even worse if the client delays writing back changes to the server. The server has an obsolete version of the file, and new file Read requests to the server may contain obsolete data. The problem of keeping local cache copies up to date to changes in remote data is known as the cache consistency problem.
The simplest approach to cache consistency is to use file-locking techniques to prevent simultaneous access to a file by more than one client. This guarantees consistency at the expense of performance and flexibility. A more powerful approach is provided with the facility is Sprite.
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Any number of remote processes may open a file for read and create their own client cache. When an open file request to a server requests Write access and other processes have the file open for Read access, the server takes two actions. It should notify the writing process and maintain cache.
