Magnetic, flash and optical storage

Overview

Recall questions on how magnetic hard disks, flash and optical discs store data, seek and rotational delay, flash wear, Blu-ray capacity, R and RW discs, and comparing the three for speed, cost, capacity, durability and typical use.

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

   How does a magnetic hard disk store data physically?

   Model answer

   Rigid platters coated with a magnetisable material spin at several thousand revolutions per minute. A read/write head on a moving actuator arm floats a tiny distance above the surface and, to write, magnetises a very small region of the coating in one of two directions to represent a 0 or a 1. To read, the head detects the changes in the magnetic field as the surface passes beneath it. The surface is organised into concentric tracks divided into sectors, and a file may be spread across many of them.

2. 2

   In mechanical terms, why is a hard disk slower to access than a solid state drive?

   Model answer

   Before any data can move, the actuator arm must swing the head to the correct track, which is the seek time, and then the disk must rotate until the required sector passes under the head, which is the rotational latency. Both are mechanical delays measured in milliseconds, and a file scattered across the disk multiplies them. A solid state drive has no moving parts and selects any block electronically, so it reaches any address in roughly the same very short time no matter what it read last.

3. 3

   How does flash storage hold data with the power switched off?

   Model answer

   Each cell is a transistor with an extra floating gate wrapped in an insulating layer. Writing forces electrons through that insulation onto the floating gate, where the insulation traps them; they stay put indefinitely with no power applied, which is what makes flash non-volatile. The presence or absence of that trapped charge changes the voltage needed to switch the transistor on, and measuring that is how the stored bit is read back.

4. 4

   What is the main wear limitation of flash storage, and how is it managed?

   Model answer

   Every erase and rewrite forces electrons through the insulating layer and degrades it slightly, so a cell can only be rewritten a finite number of times before it stops holding charge reliably. Flash also has to erase a whole block before any part of it can be rewritten, which multiplies the wear. Drive controllers deal with this by wear levelling, deliberately spreading writes evenly across all the blocks rather than repeatedly hammering the same ones, and by keeping spare blocks to substitute for any that fail. The drive therefore lasts far longer than any individual cell would suggest, though it is a genuine limit for very write-heavy work.

5. 5

   How does an optical disc store data?

   Model answer

   Data is laid out along a single spiral track running from the centre outwards. It is recorded as a pattern of pits, either pressed in during manufacture or burned by a laser, with the flat areas between them called lands. To read, a laser is shone at the track and a photodiode measures how much light comes back; a pit and a land reflect differently, and the transitions between them are interpreted as the bit pattern.

6. 6

   Why does a Blu-ray disc hold so much more than a CD of the same size?

   Model answer

   Because of the wavelength of the laser. A blue-violet laser at around 405 nanometres can be focused to a much smaller spot than the infrared laser of around 780 nanometres used for CDs, so the pits can be made smaller and the turns of the spiral packed closer together, fitting far more of them onto the same physical area. Multiple recording layers add more capacity again. Typical single-layer figures are about 700 MB for a CD, 4.7 GB for a DVD and 25 GB for a Blu-ray.

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