How (Hard) Disks Work

• The disk surface is circular and coated with magnetic material. The disk is always spinning.

• Tracks are concentric rings on the disk with bits laid out serially on the tracks.

• Each track is split into sectors, the minimum unit of transfer from the disk.

• Disks come organized in disk packs consisting of a stack of platters. Disk packs use both sides of the platters, except on the ends.

• The Comb has two read/write assemblies on the end of each arm.

• Cylinders are vertical slices through the disk pack.

• Disk operations are in terms of radial coordinates.
  1. Move arm to correct track, and wait for the disk to rotate under the head.

  2. Select and transfet the correct sector as it spends by.

Some definitions:

• Overhead - the time the CPU takes to start a disk operation.

• Latency - the time to initiate a disk transfer of one byte to/from memory. Latency consists of the time it takes to position the head over the correct cylinder (seek time) plus the time for the correct sector to come under the head (rotational delay).

• Bandwidth - once a transfer is initiated, the rate of I/O transfer (in bytes/second).

• Seek - position the head over the correct track. The seek time depends on how fast the hardware moves across the arm. Typically, it takes 10 msec to move the arm across the entire disk,

• Rotational delay - the time for the correct sector to move under the head. Typically, disks rotate at 120 cps, so a full rotation takes 8 msec.

• Transfer time - the time to move the bytes from disk to memory. A typical transfer rate is 4 MB/sec. For a 1 KB sector, transfer time is .25 msec.

• Disk I/O time = seek + rotational delay + transfer time.

• If the head is positioned at a random place on the disk, then
  1. average seek time is 5 msec.

  2. average rotational delay is 4 msec.

  3. transfer rate is .25 msec.

  4. I/O time is 9.25 msec, or roughly 100 KB/sec.

• If the random destination is one the same cylinder as the start, then no seek is needed, cutting I/O time to roughly 5 msec to fetch or put 1 KB of data. In other words, 200 KB/sec.

• If the destination is the next sector, then we have no rotational delay either, reducing I/O time to .25 msex, or 4 MB/sec.

How do we select a sector size?

• We need to synchronize the head with rotation speed.

• We need about 100 bits between sectors to tolerate small changes in the track length.

1. What if sector is 1 byte?
   • Space efficiency is terrible - only about 1% of disk is useful data.

   • Time efficiency is terrible - each seek takes 10 msec, and the transfer rate is only about 100 B/sec.

2. What if a sector is 1 KB?
   • Space efficiency is reasonable - 90% of the disk is useful data.

   • Time efficiency is 100 KB/sec.

3. What if a sector is 1 MB?
   • Space efficiency is great - most of disk contains useful data.

   • Time efficiency is great - 4 MB/sec transfer rate.

   • But 1 MB blocks are really wasteful if we only need to store a few bytes.

Some typical figures: