Magnetic Disks

• Magnetic disks provide the bulk of secondary storage for modern computer systems. Conceptually, disks are relatively simple (see Fig. 1).

  Figure 1: Moving-head disk mechanism.

  
  • Each disk platter has a flat circular shape, like a CD.
  • Common platter diameters range from 1.8 to 5.25 inches.
  • The two surfaces of a platter are covered with a magnetic material. We store information by recording it magnetically on the platters.
  • A read-write head ``flies'' just above each surface of every platter.
  • The heads are attached to a disk arm that moves all the heads as a unit.
  • The surface of a platter is logically divided into circular tracks, which are subdivided into sectors.
  • The set of tracks that are at one arm position makes up a cylinder.
  • There may be thousands of concentric cylinders in a disk drive, and each track may contain hundreds of sectors.
• When the disk is in use, a drive motor spins it at high speed. Most drives rotate 60 to 200 times per second.
• Execution of a disk operation involves (see Fig. 2)

  Figure 2: Disk Performance.

  
  1. 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.
  2. Positioning time: time hardware need to position the head. Sometimes called the random-access time,
     • consists of the time to move the disk arm to the desired cylinder, called the seek time $T_s$ (in milliseconds),
     • the time for the desired sector to rotate to the disk head, called the rotational latency (in milliseconds). Rotational speed, $r$, of 5000 to 10000 rpm.
  3. Transfer rate is the rate at which data flow between the drive and the computer (megabytes of data per second). 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 for $T_s=2$ ms, $r=10000$rpm, 512B sector size, 320 sectors per track, 1.3 MB file size.
  • At 10000 rpm, one revolution per 6ms$\Rightarrow$ average delay 3ms (=(60 second/10000) & $\frac{1}{r}=6$).
  • Read a file with 2560 sectors (=(1.3MB/512))
  • File stored compactly (8 adjacent tracks (=(2560/320))). Read first track;
    

    Average seek	2ms
    Rot. Delay	3ms
    Read 320 sectors	6ms ($\frac{1}{r}=6$, b=512*320 & N=512*320)
    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 (=(6/320); $\frac{1}{r}=6$, b=512 , N=512*320 )
    Total	5.01875ms
    All	2560*5.01875=12848ms

• Disk Performance is entirely dominated by Seek and Rotational Delays.
  • It 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.
• Floppy disks are inexpensive removable magnetic disks that have a soft plastic case containing a flexible platter.
  • The head of a floppy-disk drive generally sits directly on the disk surface, so the drive is designed to rotate more slowly than a hard-disk drive to reduce the wear on the disk surface.
• A disk drive is attached to a computer by a set of wires called an I/O bus.
• Several kinds of buses are available, including
  • enhanced integrated drive electronics (EIDE),
  • advanced technology attachment (ATA),
  • serial ATA (SATA),
  • universal serial bus (USB),
  • fiber channel (FC),
  • SCSI buses
• The data transfers on a bus are carried out by special electronic processors called controllers.
  • The host controller is the controller at the computer end of the bus.
    • To perform a disk I/O operation, the computer places a command into the host controller (memory-mapped I/O ports).
    • The host controller then sends the command via messages to the disk controller.
    • The disk controller operates the disk-drive hardware to carry out the command.
  • A disk controller is built into each disk drive.
    • Disk controllers usually have a built-in cache.
    • Data transfer at the disk drive happens
      • between the cache and the disk surface,
      • between the cache and the host controller.

  Figure 3: System's partition table.