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Secondary storage – magnetic, optical and solid-state devices compared

Computer ScienceFiles, Storage & DatabasesAges 16–17

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Look inside three kinds of storage: a hard disk drive (spinning platters, magnetised regions, read/write head, seek time and rotational latency), an optical disc (pits and lands read by a laser; CD, DVD, Blu-ray) and an SSD (NAND flash cells with a floating gate, wear levelling). Compare the capacity, speed, cost per GB, durability and portability of HDD, SSD, USB flash drive, memory card, Blu-ray disc, magnetic tape and cloud storage, choose a device for each scenario, and see virtual memory in the memory hierarchy.

Lesson: Secondary storage: magnetic, optical and solid-state devices, cloud storage, virtual memory and the memory hierarchy

What it shows

Secondary storage keeps data when the power is off. A hard disk stores each bit as a magnetised region on a spinning platter; to read, the head must seek to the right track and then wait for the sector to rotate round. An optical disc stores bits as pits and lands on a spiral track, read by the change in reflected laser light; shorter wavelengths fit more data. An SSD traps electrons on the floating gate of NAND flash cells, has no moving parts, and spreads its writes to slow down wear. No device is best for everything, so choices weigh capacity, speed, cost, durability and portability.

How to use

On How bits are stored, pick Hard disk, Optical disc or SSD and set the Byte to store. For the hard disk choose a Rotation speed and press Read a random sector or tap the platter; for discs change Disc type; for the SSD switch Wear levelling and press Keep saving. On Compare and choose, pick a Criterion, a Scenario and Your choice, then press Check. On Memory hierarchy, set RAM, open apps and switch between them.

Parameters you can change

  • Opening screen How bits are stored, Compare and choose, Memory hierarchy
  • Device shown inside Hard disk (HDD), Optical disc, SSD (NAND flash)
  • Byte to store (0–255) 0–255
  • Hard disk rotation speed 5400 rpm, 7200 rpm, 15 000 rpm
  • Optical disc type CD, DVD, Blu-ray
  • SSD wear levelling on
  • Comparison criterion Typical capacity, Read speed, Access time, Cost per GB, Durability, Portability
  • Scenario Back up 1.5 TB of family photos and videos, Built-in storage of a smartphone, Keep 500 TB of company records for 20 years, Main drive of a gaming laptop, Sell a film to customers on a disc, Carry 2 GB of schoolwork between school and home
  • RAM size 4–32 GB
  • Virtual memory on SSD, HDD

Questions to explore

  1. Why does a 15 000 rpm hard disk have a shorter average rotational latency than a 5400 rpm disk?
  2. How does a shorter laser wavelength let a Blu-ray disc hold more data than a DVD?
  3. Why does switching to an app become slow when its data is in virtual memory on a hard disk?

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