Improving file system performance by dynamically restructuring disk space

As files are added to and deleted from a file system, the disk becomes fragmented, much as main memory becomes fragmented, because of dynamic storage management. After a period of time, individual files tend to be spread across the disk rather than stored in contiguous areas. When this happens, file access times increase and disk throughput decreases. One possible solution to this problem involves disk restructuring techniques: the constituent disk blocks of selected files are moved so that total seek time for these files is decreased. Techniques to determine which files should be restructured based on the principle of least effort are introduced, and the results of experiments on a 4.2 BSD Unix file system are presented. The experiments demonstrated that over a two-week period seek time performance degraded by more than 20% as a result of fragmentation, despite attempts to control it through careful allocation.<>