“An Introduction to Disk Drive Modeling” - Ruemmler and Wilkes

Question: How good are disk drive simulation models? Result:

Demonstrates and describes a calibrated, highquality disk drive model in which the overall error factor is 14 times smaller than that of a simple first order model

Trends

• 40-60% microprocessor speedups / year

Modern Drive Characteristics

• State of the art in nonremovable magnetic disk drives with SCSI (small computer systems interconnect) controllers
  • Disk Drives have:
    • Mechanism: Recording components (disks / heads) and positioning components (assembly that moves heads & track following system)
    • Controller: Manages storage and retrieval of data from mechanism and performs logical address -> physical sector mappings

      In Depth time

      #####Recording:

  • Disk Sizes: 1.3-8” diameter [2.5, 3.5, 5.25 common]
  • Smaller disk = less {surface area, data}, less{ power, seek distance}, spin faster
    • As storage density increases, people prefer smaller disks
  • Increased storage density means:
    • Better Linear Recording Density : 50k bits/pinch. Determined by max rate of flux changes
    • Packing Data Tracks closer: 2.5k tracks/inch
  • Single disk has 1-12 platters spinning in lockstep
  • 3600-7200 rpm
  • Single r/w data channel that switches between heads that encodes/decodes data stream to magnetic phase changes
  • Multichannel disks are costly because of cross talk between concurrently active channels and keeping heads aligned simultaneously #####Positioning:
  • Data stored as concentric circles (tracks)
  • Stack of tracks $k away on all platters is a cylinder
    • 3.5 inch disk ~ 2k cylinders
  • Data accessed by disk head over track
  • All disk arms are attached to same rotation pivot
  • Positioning system ensures head gets to the desired track and stays there no matter what external factors: e.g. vibrations, shocks, disk flaws
  • Seek time ~ power^2 & Arm’s Stiffness (30-40g accelerations to achieve good seek times, and flexible arms can twist)

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