Книга: Distributed operating systems

5.3.4. Mobile Users

5.3.4. Mobile Users

Portable computers are the fastest-growing segment of the computer business. Laptop computers, notebook computers, and pocket computers can be found everywhere these days, and they are multiplying like rabbits. Although computing while driving is hard, computing while flying is not. Telephones are now common in airplanes, so can flying FAXes and mobile modems be far behind? Nevertheless, the total bandwidth available from an airplane to the ground is quite low, and many places users want to go have no online connection at all.

The inevitable conclusion is that a large fraction of the time, the user will be off-line, disconnected from the file system. Few current systems were designed for such use, although Satyanarayanan (1990b) has reported some initial work in this direction.

Any solution is probably going to have to be based on caching. While connected, the user downloads to the portable those files expected to be needed later. These are used while disconnected. When reconnect occurs, the files in the cache will have to be merged with those in the file tree. Since disconnect can last for hours or days, the problems of maintaining cache consistency are much more severe than in online systems.

Another problem is that when reconnection does occur, the user may be in a city far away from his home base. Placing a phone call to the home machine is one way to get resynchronized, but the telephone bandwidth is low. Besides, in a truly distributed system contacting the local file server should be enough. The design of a worldwide, fully transparent distributed file system for simultaneous use by millions of mobile and frequently disconnected users is left as an exercise for the reader.

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