1.2.2 Secondary Storage

Secondary storage

It's non-volatile (holds data even when system is turned off) and much bigger than primary.

Used for long term storage of data and files

The OS, system software and application software are all installed here, plus files and folders.

This type of storage cannot be read directly by the CPU.

Examples are Hard Disk, USB Flash Drive, SSD, CD

Why is secondary storage needed

Long term storage of files e.g. operating system/applications

Even when the system is switched off / it is non-volatile.

Secondary storage considerations

Capacity/size

Speed

Portability

Durability

Reliability

Cost

Remember this...

Computer Science Pupils Do Really Care

Secondary storage scenario example - Digital camera

Portable device

Lightweight

…e.g. device needs to be carried

Small physical size

…e.g. can fit in a small camera

Durable

...No moving parts

…e.g. device is moved so may be dropped // won’t be damaged when moving around

Reliable

…e.g. needs to work when out in the ‘field’

Sufficient/large capacity

...Videos are large file size // store more videos

Fast access/read/write speed

…e.g. the device will retrieve the videos without delay

Efficient power consumption

…e.g. run on battery // longer battery life

Optical

Secondary storage that uses laser light to read/burn data onto a medium such as CD, DVD or Blu-Ray disc

Magnetic

Secondary storage type stores data by magnetising areas of the surface

It includes hard disk drives and tape drives

Solid state

Secondary storage type with no moving parts

It uses semiconductor technology, and works by trapping electrons inside a gate.

Includes SSDs, flash memory, memory sticks and SD cards

Capacity

The amount of data that can be stored in a storage device or on storage media.

If you create lots of data you need this characteristic to be high

Read/Write Speed

How fast data can be read from or written to a storage device.

This characteristic is important if your application creates or uses a lot of data quickly such as a video camera

Portablility

How easy it is to carry around a storage device or its media.

You need this characteristic for secondary storage in smartphones, laptops and tablets

Durability

How well it survives knocks and drops.

A device that is carried around a lot needs this characteristic to be good

Reliability

How long a device will last without losing data...

...under normal use

Cost per byte

How expensive a device is for each byte stored

Some devices are faster and more durable but that makes the cost higher e.g. SSDs

Volatile

Loses its contents when switched off, this describes main memory aka RAM.

Because data in RAM is not permanent, we need secondary storage.

Non-volatile

Retains its contents when switched off, like all secondary storage so we can save data and install programs permanently

Magnetic storage benefits

Cheaper for larger amounts of storage space (cheaper per GB)

Greater longevity (reliability) for read/write functions.

Could be better option if increased read and write speed is not a priority

Magnetic storage drawbacks

Lots of mechanical parts so durability an issue...

...not very portable for this reason

Can be noisy when reading/writing data

Read and write speeds lower than solid state (although faster than optical)

Optical storage benefits

Cheap per byte // cheap to manufacture

Takes up little space physically...

...so is very portable

Useful for archiving data

Optical storage drawbacks

Less storage capacity compared to other types

Easily damaged / scratched,

Requires a disk reader

Slow write speeds compared to other types

Some types can only be written to once e.g. DVD-R

Magnetic storage uses

Storage of vast quantities of data

Back-ups/archiving

Optical uses

Backup and archiving of data

Optical media comparison

Solid state drive benefits over HDD

•Don't need to "get up to speed" (platters/disks) to work properly/no latency

•Lower/less power consumption/more energy efficient

•Runs cooler

•Runs quieter

•Data access is faster

•Occupies less physical space/more compact

•Lighter, so more suitable for a portable computer/laptop

•No moving parts so more reliable/durable in a portable computer/laptop

Solid state drawbacks

More expensive per GB

Generally less storage capacity in comparison to HDD

Memory cells can degrade over time...

...leading to reduced lifespan

Solid state uses

Smart phones/tablets/camera/USB drives

Operating systems and other applications are installed on them for fast read speeds

Memory card

A solid state storage device with a flat shape that is used in cameras and phones, also known as a flash card or SD card

Memory Stick

A solid state storage removable storage device which is very portable, comes in capacities from 1GB up to 512GB

Solid State Drive

No moving parts means this high capacity, fast, durable, low power and reliable secondary storage device is good for laptops but it's expensive

Blu-Ray Disc

An optical storage component with a capacity of 25GB, a blue laser reads them, they are more expensive than DVDs but still cheap to make.

DVD

Optical storage, a laser reads and writes the 4.7GB capacity optical disks which are cheap and portable, but a bit more expensive than CDs.

CD

Optical storage, a laser reads and writes the 700MB capacity optical disks. Disks are cheap and portable.

Hard Disk Drive

High capacity magnetic secondary storage device. It has spinning platters so it wears out, making it less reliable and portable than SSD.