Secondary storage, fragmentation and defragmentation

Overview

Track, sector and cylinder on a hard disc and consequences of heavy fragmentation on a magnetic hard disc.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why does a computer need secondary storage when it already has main memory?

   Model answer

   Secondary storage keeps its contents when the power is off, offers far greater capacity and costs much less per byte, so programs and files survive between sessions.

2. 2

   How does a magnetic hard disc store a single bit?

   Model answer

   A small patch of the coating on a spinning platter is magnetised in one of two directions, and the direction of that magnetisation represents a 0 or a 1.

3. 3

   Define track, sector and cylinder on a hard disc.

   Model answer

   A track is one concentric ring on a surface, a sector is a fixed-size division of a track and is the smallest block that can be read or written, and a cylinder is the set of tracks at the same radius on every surface of the stack.

4. 4

   Name the two delays that make up the access time of a hard disc.

   Model answer

   Seek delay, while the head moves to the correct track, and rotational delay, while the wanted sector spins round to reach the head.

5. 5

   How does a solid state drive store data?

   Model answer

   It stores each bit as trapped charge in a flash memory cell, and because it holds charge rather than moving anything, it has no moving parts at all.

6. 6

   Give three advantages of a solid state drive over a magnetic hard disc.

   Model answer

   It reads and writes far faster because there is nothing to move into position, it is silent and uses less power, and it survives being knocked or dropped.

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