Disk Hardware

• Disk drives addressed as large 1-dimensional arrays of logical blocks (smallest transfer unit)
• 1-dimensional array of logical blocks mapped onto sectors of disk sequentially
  • sector 0: 1st sector of 1st track on outermost cylinder
  • mapping in order through that track, then rest of tracks in that cylinder, then through rest of cylinders from outermost to innermost
• Outer tracks can store more sectors than inner without exceed max information density (see Fig. 5.14 Left)
• Evolution of Disk Hardware (see Fig. 5.14 Right)

  Figure 5.14: Left: (a) Physical geometry of a disk with two zones (b) A possible virtual geometry for this disk, Right: Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk.
  • Average seek time is approx 12 times better
  • Rotation time is 24 times faster
  • Transfer time is 1300 times faster
  • Most of this gain is due to increase in density
  • Represents a gradual engineering improvement
• Disk Performance (see Fig. 5.15)
  • Disk is a moving device; must be positioned correctly for I/O
  • Execution of a disk operation involves
    • Wait time: the process waits to be granted device access
      • Wait for device: time the request spend in wait queue
      • Wait for channel: time until a shared I/O channel is available
    • Access time: time hardware need to position the head
      • Seek time: position the head at the desire track
      • Rotational delay (latency): spin disk to the desired sector
    • Transfer time: sectors to be read/written rotate below head

  Figure 5.15: Disk Performance.

  
• Estimating Access Time;
  • Seek Time $T_s$: Moving the head to the required tgrack not linear in the number of tracks to traverse: startup time, settling time. Typical avearge seek time: a few milliseconds
  • Rotational delay: rotational speed, $r$, of 5000 to 10000 rpm. At 10000 rpm, one revolution per 6ms $\Rightarrow$ average delay 3ms
  • Transfer time: to transfer $b$ bytes, with $N$ bytes per track;
    $$T={b \over rN}$$
    Total average access time:
    $$T_a=T_s+{1 \over 2r}+{b \over rN}$$

• A Timing Comparison
  • $T_s=2$ ms, $r=10000$rpm, 512B sect, 320 sect/track
  • read a file with 2560 sectors (=1.3MB)
  • file stored compactly (8 adjacent tracks): Read first track
    

    Average seek	2ms
    Rot. Delay	3ms
    Read 320 sectors	6ms
    Total	11ms
    All sectors	11+7*9=74ms

  • Sectors distributed randomly over the disk: Read any sector
    

    Average seek	2ms
    Rot. Delay	3ms
    Read 1 sectors	0.01875ms
    Total	5.01875ms
    All	2560*5.01875=20,328ms

• Disk Performance is Entirely Dominated by Seek and Rotational Delays
  • Will only get worse as capacity increases much faster than increase in seek time and rotation speed (it has been easier to spin the disk faster than improve seek time)
  • Operating System should minimise mechanical delays as much as possible