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Horst F. Wedde, G. S. Alijani, D. Baran-Huizinga, G. Kang und B. K. Kim
Real time file performance of a completely decentralized adaptive file system
Proc. of the Int. IEEE Symposium on Real-Time Systems, Santa Monica, CA, 1989-12


Described are the major features of the completely decentralized adaptive file system Melody, which runs on top of the distributed operating system Dragon Slayer. A definition of all major design requirements is presented, along with a heuristic algorithm for resource scheduling which allows for replicating, relocating, or deleting file copies in a local area network. At every site a local task scheduler tries to schedule the arriving critical tasks such that no deadlines are missed, based on the availability of resources at this site. Depending on the task-deadline failure history and file-access request history at each node, file copies may be relocated, replicated to other sites, or deleted. Such copies may also assume a different status (public or private) depending on how recent their information is. Only public copies are maintained under a strong concurrency-control protocol. The idea is to maintain optimal distribution of file copies in the network regarding reliability and real-time responsiveness of the system. Experiments performed to analyze Melody's capability of minimizing deadline failures of time-critical tasks were compared to structurally simpler file-system models. A report is presented on the distributed simulation results which show Melody's superior performance and its built-in sensitivity to changes in the environment.