The need for secondary storage
Secondary storage holds programs and data permanently, when they are not being used.
Why computers need it comes down to two limitations of RAM. RAM is volatile, so everything in it is lost when the power goes off — without secondary storage, a computer would forget every file and program every time it was switched off. And RAM is expensive per gigabyte, so a computer could not afford enough of it to hold everything a user owns.
Secondary storage solves both: it is non-volatile and far cheaper per gigabyte, at the cost of being much slower.
- Secondary storage
- storage that holds programs and data permanently when they are not being used; it is non-volatile, so its contents survive the power being switched off
The three common types
Knowledge of the component parts of these types of storage is not required, so you do not need to know what is inside an SSD — only how each type behaves against the six characteristics.
Choosing storage for a scenario
The specification asks you to apply this knowledge in context, which means recommending a device for a described situation and justifying it against the characteristics that matter there. The characteristics do not all matter equally in every case — the skill is deciding which ones do.
The questions to ask:
• How much data? If it is measured in terabytes, optical is out. • Does it need to be carried around? If so, portability and durability matter more than capacity. • Will it be knocked or dropped? Solid state, because there are no moving parts. • Does speed matter? If the computer boots from it or it holds a large program, solid state. • Is cost the priority? Magnetic gives the most gigabytes per pound. • Does it need to be given to someone, or read on many machines? Optical, or a USB stick.
Worked example
A field researcher needs storage for photographs, carried in a rucksack over rough terrain, with a limited budget. Recommend a type and justify it.
- Identify which characteristics matter here: it will be carried and knocked about, so durability and portability come first.
- Rule out magnetic: an HDD has moving parts and is easily damaged by being dropped or jolted.
- Rule out optical: capacity is far too low for a large number of photographs, and discs scratch.
- Solid state has no moving parts, so it survives knocks, and it is light and small.
- Address the budget, since the question raised it.
Answer: Solid state — a portable SSD or memory cards. With no moving parts it withstands being knocked about in a rucksack, and it is light and compact. It costs more per gigabyte than magnetic, but a magnetic drive would be likely to fail in these conditions, so the extra cost buys reliability where it matters most.
Common mistake
Answer against the scenario, not in general. "SSDs are fast" earns little if the question is about carrying photographs across a field — the marks are for identifying which characteristics matter in that situation and why.
Before you practise
- Secondary storage is needed because RAM is volatile and too expensive to hold everything.
- The three types are optical, magnetic and solid state.
- Optical: cheap and very portable, but low capacity, slow and easily scratched.
- Magnetic: highest capacity for the money, but has moving parts that wear out and are damaged by knocks.
- Solid state: fast, durable and portable with no moving parts, but the most expensive per gigabyte.
- The six characteristics to compare: capacity, speed, portability, durability, reliability and cost.
- Knowledge of the component parts of each type is not required.
- In a scenario question, say which characteristics matter there and justify against those.