On the effectiveness of buffered and multiple arm disks

Access patterns to input/output files are usually sequential and local; a small number of files are open for each job and the file is generally read or written sequentially. This implies that if files are allocated in contiguous disk storage, the disk arms will rarely be required to move. Disk seeks can be almost entirely eliminated by using multiple arms, whereby if there are several open files on a given spindle, there will be an arm for each. Even better results can be obtained by using electronic storage such as magnetic bubbles, CCD's or electron beam memories to buffer portions of the disk. Such buffers not only eliminate seek time but reduce rotational latency to negligible amounts and decrease transfer time. We study, using trace driven simulation, the effectiveness of both buffers and multiple arms. For the data used, our experiments show that a minimum total buffer capacity of 20 tracks is required in a buffered disk. Best hit ratio results are obtained with individual buffer sizes upwards of one quarter cylinder. Three cylinder size buffers give very high hit ratios, on the order of 96%, and this appears to be a good choice for buffer size and number. LRU buffer scheduling is found to work well; comparisons with optimal, look-ahead buffer scheduling are made. Some possible models for the frequency of seeks are briefly considered.