Secondary Storage
Why every computer needs secondary storage, the three technology types, and how to choose the right device using the six characteristics examiners expect.
Why secondary storage is needed
RAM is volatile. When the power goes, everything in it is gone. So a computer needs somewhere to keep data permanently, and that is secondary storage.
Three reasons a computer needs it:
- Persistence. Files, programs and the operating system must survive being switched off.
- Capacity. RAM is expensive per gigabyte. Secondary storage gives far more space for the money.
- Portability and backup. Data can be moved between machines or kept as a safe copy.
Secondary storage is non volatile, so it retains data when the computer is switched off, which primary storage cannot do.
The three types
Magnetic
A hard disk drive (HDD) contains rigid platters coated in magnetic material, spinning at high speed. A read write head on an arm moves across the surface, magnetising tiny regions to represent 1s and 0s.
- High capacity for low cost per gigabyte
- Slower than solid state, because parts must physically move
- Contains moving parts, so it is vulnerable to being knocked or dropped
- Makes noise and generates heat
- Very long lasting if left undisturbed
Magnetic tape is also magnetic. It is extremely cheap per terabyte and still used for large scale archive backups, but access is serial, so reaching data at the end means winding through everything before it.
Solid state
A solid state drive (SSD) stores data in flash memory using transistors that hold a charge. There are no moving parts at all.
- Much faster read and write speeds than magnetic
- Silent, and produces less heat
- Uses less power, which matters for battery life
- More resistant to being dropped
- More expensive per gigabyte
- Has a finite number of write cycles, though modern drives outlast typical use
USB flash drives and memory cards use the same flash technology.
Optical
CDs, DVDs and Blu-ray discs store data as physical pits and lands burned into a reflective layer, read by a laser.
| Format | Typical capacity |
|---|---|
| CD | 700 MB |
| DVD | 4.7 GB single layer |
| Blu-ray | 25 GB single layer |
- Very cheap per disc and extremely portable
- Low capacity by modern standards
- Slow compared with SSD and HDD
- Easily scratched
- Read only versions cannot be altered, which is useful for distributing software
Even though few laptops now have a disc drive, optical storage still appears in questions about distributing software to many people cheaply, or supplying a copy that cannot be modified.
- Magnetic: spinning platters with a head floating over the surface.
- Solid state: a grid of cells holding charge, with no moving parts.
- Optical: one spiral track of pits and lands, read by a laser.
- Which one depends on capacity, speed, durability and cost per gigabyte.
Choosing the right device
Six characteristics decide which device suits a situation. Learn all six, because questions often ask for three with justification.
| Characteristic | What it means | Typical winner |
|---|---|---|
| Capacity | How much data it holds | HDD, or tape for archives |
| Speed | How quickly data is read and written | SSD |
| Portability | How easily it is carried | Flash drive, optical |
| Durability | How well it survives knocks, drops and moisture | SSD, no moving parts |
| Reliability | How likely it is to fail over time | SSD for shock, HDD for long term archive |
| Cost | Price per gigabyte | HDD, or tape, cheapest per TB |
Working through an example
A photographer needs storage for a laptop used outdoors on location.
- Durability matters most, the laptop is being carried and knocked, so an SSD with no moving parts is far less likely to be damaged.
- Speed matters, large image files must be written quickly.
- Capacity is a concern, but a 2 TB SSD is now realistic.
- Cost is the trade off: an SSD costs more per gigabyte than an HDD.
Conclusion: SSD, justified by durability and speed, accepting the higher cost.
Do not write "an SSD is better because it is good". Write "an SSD is more suitable because it has no moving parts, so it is more durable when the laptop is carried between locations".
Key terms you must be able to define
- Secondary storage
- Non volatile storage used to keep programs and data permanently, even when the computer is switched off.
- Magnetic storage
- Storage that represents data by magnetising regions of a surface, such as a hard disk drive or tape.
- Solid state storage
- Storage using flash memory with no moving parts, such as an SSD or USB flash drive.
- Optical storage
- Storage read by a laser from pits and lands on a disc, such as a CD, DVD or Blu-ray.
- Durability
- How well a storage device withstands physical damage such as being dropped.
- Portability
- How easily a storage device can be carried and moved between computers.
- Capacity
- The total amount of data a device can hold.
- Write cycle
- One operation writing data to a flash memory cell. Flash memory supports a large but finite number.
Flashcards
Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.
Reach the top grade on this topic
Nearly all the marks here are for matching a characteristic to a scenario. The knowledge is easy, the application is where students lose marks.
The technique is a three part sentence, repeated:
- Name the device.
- Name the characteristic.
- Link it explicitly to something in the question.
"An SSD is most suitable because it has no moving parts, so it is more durable when the laptop is carried between filming locations."
Watch for the trade off. Almost every scenario has one, and stating it earns the higher mark:
- SSD gives speed and durability but costs more per gigabyte
- HDD gives capacity cheaply but is fragile and slower
- Optical is cheap and portable but low capacity and easily scratched
- Tape is cheapest per terabyte for archives but is serial access, so retrieval is slow
Also know the reason each device is fast or slow. SSD is fast because there is nothing to move. HDD is slower because the platter must rotate and the head must seek to the right track.
Mistakes that cost marks here
- Recommending a device without naming which characteristic makes it suitable.
- Saying an SSD 'never fails'. Flash memory has a finite number of write cycles, and any drive can fail.
- Calling RAM a type of secondary storage. RAM is primary storage and it is volatile.
- Writing 'a hard drive' when the question is about a laptop being carried around, and ignoring the durability issue.
- Forgetting that the operating system itself is stored in secondary storage, not in ROM.
Magnetic hard disks have platters spinning at 7200 revolutions per minute with a read write head hovering roughly 3 nanometres above the surface. Scaled up, that is a jumbo jet flying at full speed a few millimetres above the ground.
Knowledge check
Exam-style questions
Five written questions in the style of the real paper. Write a full answer, then let the marker check it against the mark scheme.
- LSecondary storage is non volatile
- LSo data and programs are kept when the computer is switched off, which RAM cannot do
Secondary storage is required because it is non volatile, which means it retains its contents when the power is removed. RAM is volatile and loses everything when the computer is switched off, so files, applications and the operating system itself must be stored in secondary storage in order to survive between sessions.
- LSolid state has no moving parts, magnetic uses spinning platters and a moving read write head
- LSolid state is faster to read and write
- LSolid state is more durable and resistant to physical shock
- LMagnetic is cheaper per gigabyte and often available in larger capacities
The first difference is construction. Solid state storage uses flash memory with no moving parts, whereas magnetic storage uses rigid platters that spin at high speed with a read write head that moves across the surface. This leads directly to the second difference: because nothing has to move mechanically, a solid state drive reads and writes data significantly faster and is far more resistant to damage from being knocked or dropped. In exchange, magnetic storage is considerably cheaper per gigabyte and is typically available in larger capacities for the same price.
- LRecommends solid state storage
- LDurability: no moving parts so it survives transport and rough handling
- LSpeed: video files are very large so fast write speed matters
- LPortability: physically small and light to carry
- LPower consumption is lower, useful when mains power is unavailable
- LAcknowledges the higher cost per gigabyte as the trade off
The crew should use solid state storage such as portable SSDs or high capacity memory cards. The most important factor is durability: they are working in remote areas and equipment will be carried, jolted in vehicles and used in poor conditions, and because solid state storage has no moving parts it is far less likely to be damaged by physical shock than a magnetic hard disk with spinning platters and a floating read write head. Speed is the second consideration, because raw video files are extremely large and a slow write speed would either drop frames during recording or leave the crew waiting a long time to transfer footage at the end of a shoot. Solid state devices are also physically small and light, which matters when everything has to be carried, and they draw less power, which is a genuine benefit when mains electricity is not available and everything is running from batteries. The clear trade off is cost, since solid state storage is significantly more expensive per gigabyte than magnetic storage, so the crew will pay considerably more for the same capacity. Given that losing footage would mean losing an entire shoot that cannot easily be repeated, the extra cost is justified.
- LVery cheap to produce in large quantities
- LRead only versions cannot be modified or overwritten by the customer
- LDiscs are lightweight and easy to post or stock in shops
Optical discs remain a reasonable choice for software distribution because they are extremely cheap to manufacture in large quantities, so the cost of producing many thousands of copies is very low. Read only formats also cannot be altered or overwritten by the customer, which protects the integrity of the software and prevents accidental modification. Finally, discs are thin, light and robust enough to be posted or stocked on shop shelves easily, which makes physical distribution straightforward.
- LRecommends magnetic hard disk drives
- LCheapest cost per gigabyte, important when buying 500 units
- LLarge capacity available for documents and presentations
- LDesktop machines are not moved, so the durability weakness of magnetic storage does not matter
- LNotes that documents are small files, so the speed advantage of SSD is not worth the extra cost here
The school should choose magnetic hard disk drives. The dominant factor is cost, because buying storage for 500 machines multiplies any price difference five hundred times, and magnetic storage has by far the lowest cost per gigabyte. Hard disks also offer large capacities as standard, which comfortably exceeds what is needed for documents and presentations. The usual weakness of magnetic storage, its vulnerability to physical shock because of the spinning platters and moving head, is not relevant here because desktop computers sit permanently on desks and are not carried around. Solid state drives would boot and load faster, but student work consists of small documents and presentation files rather than large media, so the speed advantage would be barely noticeable in daily use and would not justify the substantially higher cost across the whole school.