Disk Structure

• Modern disk drives are addressed as large one-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.
• The size of a logical block is usually 512 bytes, although some disks can be low-level formatted to have a different logical block size, such as 1,024 bytes.
• The one-dimensional array of logical blocks is mapped onto the sectors of the disk sequentially.
  • Sector 0 is the first sector of the first track on the outermost cylinder.
  • The mapping proceeds in order through that track, then through the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.
• By using this mapping, we can convert a logical block number into an old-style disk address that consists of a cylinder number, a track number within that cylinder, and a sector number within that track.
• In practice, it is difficult to perform this translation, for two reasons.
  • First, most disks have some defective sectors, but the mapping hides this by substituting spare sectors from elsewhere on the disk.
  • Second, the number of sectors per track is not a constant on some drives.
• Let's look more closely at the second reason.
  • On media that use constant linear velocity (CLV), the density of bits per track is uniform.
  • The farther a track is from the center of the disk, the greater its length, so the more sectors it can hold.
    • As we move from outer zones to inner zones, the number of sectors per track decreases (see Fig. 12.4).
    • Tracks in the outermost zone typically hold 40 percent more sectors than do tracks in the innermost zone.
    • The drive increases its rotation speed as the head moves from the outer to the inner tracks to keep the same rate of data moving under the head (CD-ROM, DVD-ROM drives).

    Figure 12.4: Physical geometry of a disk with two zones and a possible virtual geometry for this disk.

    
  • Alternatively, the disk rotation speed can stay constant, and the density of bits decreases from inner tracks to outer tracks to keep the data rate constant. This method is used in hard disks and is known as constant angular velocity (CAV).
• Evolution of Disk Hardware (see Fig. 12.5)

  Figure 12.5: 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.