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Module 6: I/O & File Management

20227m

Describe seek time, rotational latency, random-access time and their relationship in context of magnetic disks by drawing its moving – head mechanism.

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Magnetic Disk Timing

  • Seek time: time for head assembly to move to correct track/cylinder (dominant, ~5-10ms). Depends on distance.
  • Rotational latency: time for desired sector to rotate under head. Average = 0.5*(60/RPM). At 7200 RPM ≈4.17ms.
  • Random-access time = Seek + Rotational latency (+ transfer time for actual R/W).
  • Transfer time = bytes / (bytes per track * RPM)

Moving-head mechanism: Platter stack with arm assembly. Diagram description: multiple platters share spindle, each surface has head on arm, all heads move together radially. Arm moves seek, then platter rotates to sector. Random time = seek + rotation. Contiguous allocation reduces seeks.

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