Disk arm movement in anticipation of future requests

  • Authors:

  • Richard P. King

  • Affiliations:

  • IBM T. J. Watson Research Center, Yorktown Heights, NY

  • Venue:
  • ACM Transactions on Computer Systems (TOCS)
  • Year:
  • 1990

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Abstract

When a disk drive's access arm is idle, it may not be at the ideal location. In anticipation of future requests, movement to some other location may be advantageous. The effectiveness of anticipatory disk arm movement is explored. Various operating conditions are considered, and the reduction in seek distances and request response times is determined for them. Suppose that successive requests are independent and uniformly distributed. By bringing the arm to the middle of its range of motion when it is idle, the expected seek distance can be reduced by 25 percent. Nonlinearity in time versus distance can whittle that 25 percent reduction down to a 13 percent reduction in seek time. Nonuniformity in request location, nonPoisson arrival processes, and high arrival rates can whittle the reduction down to nothing. However, techniques are discussed that maximize those savings that are still possible under those circumstances. Various systems with multiple arms are analyzed. Usually, it is best to spread out the arms over the disk area. The both arms should be brought to the middle.