Secondary storage devices

Overview

Recall questions on hard disks, optical disks and solid-state drives, how each stores data, what makes up access time, and which suits a given job.

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 must a computer have storage beyond its main memory?

   Model answer

   Two properties of main memory force it. First, RAM is volatile: cut the power and its contents are gone, yet the operating system, the applications and the user's files all have to still be there when the machine is switched on the next morning. Second, RAM is costly per byte and limited in how much can physically be fitted, while the data an organisation accumulates — years of records, photographs, video — runs to far more than any affordable quantity of it. Secondary storage answers both at once, since it retains its contents without power and gives far more capacity per pound. There is a third gain too: because it is separate, and sometimes removable, copies can be taken for backup, material no longer in daily use can be archived off, and data can be physically carried between machines.

2. 2

   Describe the principle of operation of a hard disk drive.

   Model answer

   One or more rigid circular platters, coated on both sides with a magnetic material, are mounted on a spindle and spun at a constant high speed, typically 5,400 or 7,200 revolutions per minute. A read/write head for each surface is carried on an actuator arm and floats a minute distance above the surface on the cushion of air dragged round by the spinning platter. To write, a current through the head's coil magnetises a tiny region of the surface in one of two directions, representing a 0 or a 1. To read, the changing magnetic fields of the regions passing beneath the head induce a small current in it, which is amplified and decoded. Because the magnetism remains when the power is off, the storage is non-volatile.

3. 3

   How is the surface of a hard disk organised, and what makes up its access time?

   Model answer

   Each surface is divided into concentric circular tracks, and each track is divided into sectors, which are the smallest units that can be read or written. The set of tracks at the same radius on all the surfaces forms a cylinder, and these can be accessed without moving the arm. Access time is made up of the seek time, the time for the actuator to move the head to the correct track; the rotational latency, the time spent waiting for the required sector to rotate under the head, on average half a revolution; and the transfer time, the time to read or write the data itself. Because the first two involve physically moving parts, they are measured in milliseconds, which is why a hard disk is enormously slower than main memory.

4. 4

   Give the main characteristics and the suitability of a hard disk.

   Model answer

   It offers very high capacity, into the tens of terabytes, at the lowest cost per gigabyte of any of these technologies, with good sustained transfer rates for large sequential files. Against that it contains fast-moving mechanical parts, so it makes noise, generates heat, consumes more power, will eventually wear out and is easily damaged by being knocked while running; and its random access is slow because the head must physically move. It is therefore well suited to bulk storage where cost per gigabyte matters most: file servers, archives, backups and large media libraries, and to desktop machines that stay still. It is poorly suited to portable and rugged equipment and to workloads dominated by small random accesses.

5. 5

   Describe the principle of operation of an optical disk.

   Model answer

   Data is held along a single spiral track running from the centre outwards, as a series of pits pressed or burned into a reflective metallic layer beneath a transparent plastic coating, with the flat areas between them called lands. A laser is focused through the plastic onto the track and the intensity of the reflected light is measured by a photodiode. Light reflected from a land returns strongly, while light striking the boundary between a pit and a land is partly cancelled by interference and returns weakly, so a transition between pit and land is read as a 1 and no transition as a 0. The disk spins while the laser assembly tracks slowly outwards.

6. 6

   What distinguishes CD, DVD and Blu-ray, and how do recordable and rewritable optical disks work?

   Model answer

   They differ chiefly in the wavelength of the laser used: infrared for CD, red for DVD and blue-violet for Blu-ray. A shorter wavelength can be focused to a smaller spot, so the pits and the spacing of the spiral track can be smaller and far more data fits on a disk of the same size, giving roughly 700 megabytes for a CD, 4.7 gigabytes for a single-layer DVD and 25 gigabytes for a single-layer Blu-ray. Pressed disks have their pits physically stamped in and are read-only. A recordable disk has a dye layer that a higher-powered laser burns to change its reflectivity permanently, so it can be written once. A rewritable disk uses a phase-change alloy that the laser can switch between a reflective crystalline state and a dull amorphous state, and back again, so it can be erased and rewritten a limited number of times.

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