An investigation of file migration in a distributed file system
Richard T. Hurley · 1992
With the demand for computer resources on the rise, distributed systems are becoming popular. Allowing more users access to resources increases the need to manage these resources efficiently. One management strategy is file migration: migrating files (and their associated requests) from congested to uncongested storage sites to improve overall system performance. In this thesis, we develop a simple, adaptive file migration policy for homogeneous distributed file systems which transfer whole files. Using simulation, we find that our policy can achieve significant performance improvements over a system without file migration that is initially balanced. There is also strong evidence that our policy is robust over a wide range of operating conditions. We begin by developing a performance model for the file transfer operation in a distributed system environment. This model incorporates such issues as flow control, prefetching and postwriting of file blocks, and the contention for buffers at the source and destination. Throughput results for the model are obtained analytically using a Markov chain analysis. These results can be used to estimate parameters for our file migration investigation. File migration is examined next. We develop a simple file migration policy based on an instantaneous performance gain analysis. Briefly, a file migration is performed whenever it is possible to lower the total response time of requests already in the system. Three models are examined. The basic model assumes that the state information used by the file migration policy is current and there is infinite storage for files. The second model investigates the effects of out-dated state information and finally, in the third model, we examine a system with storage space restrictions. Through this investigation, we are able to determine the conditions under which file migration is effective. These results can be used to develop guidelines for the design of distributed file systems.