Exercise 3: Record Layout on Disk

From DSCB 13.7.6. An MPEG movie uses about one gigabyte per hour of play. If we carefully organized several movies on a Megatron 747 disk, how many could we deliver with only small delay (say 100 milliseconds) from one disk. Use the timing estimates of Example 13.2, but remember that you can choose how the movies are laid out on the disk.

Megatron 747 characteristics:

• eight platters providing 16 surfaces
• 2**16, or 65,536 tracks per surface
• 2**8 = 256 sectors per track
• 2**12 = 4096 bytes per sector
• one block is 16,384 bytes = 4 consecutive sectors
• 7200 rpm or 8.33 millisecond per rotation
• To move the head assembly between cylinders takes one millisecond to start and stop, plus one additional millisecond for every 4000 cylinders traveled. Thus, the heads move one track in 1.00025 ms and move from the innermost to the outermost track, a distance of 65,536 tracks, in about 17.38 ms.
• gaps occupy 10% of the space around a track.

Background: ICS 321 Slides on Disk Storage

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