Better file abstraction jon used to be stored on

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Unformatted text preview: s physical storage location changes. • Better file abstraction. • ~jon– used to be stored on machine M1 now on M2 • Separates the naming hierarchy form the storage-devices hierarchy. hierarchy. Remote Remote File Access Reduce network traffic by retaining recently accessed disk blocks in a cache, so that repeated accesses to the same information can be handled locally. • If needed data not already cached, a copy of data is brought from the server to the user. Usually grab a large chunk. • Accesses are performed on the cached copy. • Files identified with one master copy residing at the server machine, but copies of (parts of) the file are scattered in different caches. • Cache-consistency problem – keeping the cached copies consistent with the master file. Cache Cache Update Policy Write-through – write data through to disk as soon as they are placed on on any cache. Reliable, but poor performance. Delayed Delayed-write – modifications written to the cache and then written through to the server later. Write accesses complete quickly; some data may be overwritten before they are written back, and so need never never be written at all. • Reliability? • Variation – scan cache at regular intervals and flush blocks that have been been modified since the last scan. Typical. • Variation – write-on-close, writes data back to the server when the file is closed. Best for files that are open for long periods and frequently frequently modified. Andrew File System (AFS). Consistency Consistency Is locally cached copy of the data consistent w...
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