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The 4.4BSD file system includes a new algorithm for allocating disk
blocks to files. The goal of this algorithm is to improve file
clustering, increasing the amount of sequential I/O when reading
or writing files, thereby improving file system performance. In
this paper we study the effectiveness of this algorithm at reducing
file system fragmentation. We have created a program that artificially
ages a file system by replaying a workload similar to that experienced
by a real file system. We used this program to evaluate the
effectiveness of the new disk allocation algorithm by replaying ten
months of activity on two file systems that differed only in the
disk allocation algorithms that they used. At the end of the ten
month simulation, the file system using the new allocation algorithm
had approximately half the fragmentation of a similarly aged file
system that used the traditional disk allocation algorithm. Measuring
the performance difference between the two file systems by reading
and writing the same set of files on the two systems showed that
this decrease in fragmentation improved file write throughput by
20% and read throughput by 32%. In certain test cases, the new
allocation algorithm provided a performance improvement of greater
than 50%.
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