File migration [Thesis]
Edward Stritter, USDOE · 1977
In a computer system, program and data files are stored on secondary memory such as discs. If the capacity of the secondary memory is not sufficient to hold all the files, some sort of back-up memory (e.g., magnetic tape) is used to supplement secondary storage. The transfer of files between secondary and back-up storage is referred to here as ''migration.'' Migration may be done explicitly by each user or automatically by the operating system. This work studies automatic migration strategies. A migration strategy is sought which maximizes the probability that a file that is accessed will be found in secondary memory and which minimizes the migration activity. Trace data of file system activity on a general purpose computer system collected for one year provide the basis for a quantitative study of migration. Various possible migration strategies are presented. Two non-equivalent criteria are used for evaluating strategies. LRU (least recently used) and MIN (the theoretically optimum strategy for fixed sized pages) are studied. Other strategies, including LFU (least frequently used), that are suggested by the results of statistical analysis are examined. The fact that files are not equal in size can be used to advantage, and replacement strategies are shown to be improved when the size of a file is taken into consideration. Finally, some non-realizable strategies using knowledge of files' future accesses are presented for comparison with the practical strategies already mentioned. A simulation model of file system activity is examined. The model is useful for generating artificial trace data. Simple parameter changes allow the model to simulate other environments. 31 figures.