Книга: Distributed operating systems



The heart of any distributed system is the distributed file system. The design of such a file system begins with the interface: What is the model of a file, and what functionality is provided? As a rule, the nature of a file is no different for the distributed case than for the single-processor case. As usual, an important part of the interface is file naming and the directory system. Naming quickly brings up the issue of transparency. To what extent is the name of a file related to its location? Can the system move a single file on its own without the file name being affected? Different systems have different answers to these questions.

File sharing in a distributed system is a complex but important topic. Various semantic models have been proposed, including UNIX semantics, session semantics, immutable files, and transaction semantics. Each has its own strengths and weaknesses. UNIX semantics is intuitive and familiar to most programmers (even non-UNIX programmers), but it is expensive to implement. Session semantics is less deterministic, but more efficient. Immutable files are unfamiliar to most people, and make updating files difficult. Transactions are frequently overkill.

Implementing a distributed file system involves making many decisions. These include whether the system should be stateless or stateful, if and how caching should be done, and how file replication can be managed. Each of these has far-ranging consequences for the designers and the users. NFS illustrates one way of building a distributed file system.

Future distributed file systems will probably have to deal with changes in hardware technology, scalability, wide-area systems, mobile users, and fault tolerance, as well as the introduction of multimedia. Many exciting challenges await us.

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