Secondary storage GCSE Computer Science MrGoff.com

Magnetic storage

Magnetic storage refers to hard disk drives. These consist of large round platters as seen in the image on the left. Data on the platters is written in concentric circles known as tracks. These tracks are divided into sectors. A file may take up several sectors. A group of contiguous sectors i.e. ones that are next to each other. These are known as clusters. The layout of a platter can be seen on the right.

On the left you can also see the actuator arm. This can move in and out and the platters themselves spin very quickly(approx 10000rpm). This means that the read right head that is on the very end of the actuator arm can reach any sector on the disk.

To write to a magnetic disk the read write head polarises a part of the disk either North or South and when reading the disk it reads these as 1 or 0.

Optical storage

Optical storage refers to CDs, DVDs and Blue ray disks. Unlike magnetic storage, optical storage consists of a single track that spirals out from the center of the disk.
To write to an optical disk a strong laser is used to burn pits into the surface of the disk. The unburned sections are known as lands. To read the disk a weak laser is shone at the surface of the disk.

A common misconception is that pits and lands themselves represent 0s and 1s. The reality is that a 0 or 1 is recorded depending on whether the weak laser reflects directly back to where it came from. It will do this if it hits a land or the center of a pit. It will not do this when the track is entering or leaving a pit as these areas are curved.

The reason that CDs, DVDs and Blue ray disks are all the same physical size but contain different amounts of data is because at each step up a finer laser is used meaning a longer spiral can be fit on the disk with more pits and lands.

Solid state storage

Solid state memory, also known as flash memory, consists of transistors that don't lose their state when the power is turned off. To set the state of a transistor in flash memory we give it a charge. This causes electrons to flow from source to drain. A positive charged is applied from above and some elctrons pass through the floating gate. They are trapped in the next section because another layer prevents them from continuing. These electrons will stay there even when the power is removed. To flush this transistor and make it back into a 0 a negative charge is applied from above and the elctrons pass back through the floating gate and head to the drain.

Comparing secondary storage methods

Consideration

Discussion

Cost

Optical disks are very cheap. Magnetic storage is considerably cheaper than solid state storage for the same capacity but its costs are decreasing fast.

Capacity

Optical disks do not hold much data with CDs holding less than a GB and DVDs at about 4.7GB and dual sided Blue ray at 50GB. Magnetic storage is available in very big sizes. Personal hard drives are regularly upwards of 1TB. Solid state drives are increasingly available in larger and larger sizes, however the bigger the size the larger the price difference between solid state and magnetic.

Speed

Optical disks are very slow to write to and fairly slow in terms of read speed. Magnetic storage is faster but because it involves the disk physically spinning and the read write head moving it is not as fast as solid state.

Portability

Optical disks are fairly portable in small cases. Traditional hard drives are not portable and easily damaged by being moved. Improvements in technology mean that some new portable hard drives can be transported relatively safely in a laptop bag or even a pocket. Solid state drives are very portable. Due to having no moving parts they are not damaged by being carried around making them ideal in things like phones.

Durability

Optical disks will generally last several years if well looked after. Magnetic storage is generally durable but may be damaged if moved while spinning. In addition it can be affected by magnets and heat. Solid state is not affected by magnets, heat or knocks. One advantage magnetic has over solid state is that you can write to a magnetic disk an unlimited number of times where as solid state does have a limited number of times it can be flashed, although that is quite a lot of times.

Reliability

Optical disks are easily scratched or damaged by sunlight. Magnetic storage and solid state storage are highly reliable.

Cloud storage

Cloud storage is storing your data in an offsite location usually with another provider. This will mean that your data is accessible via the internet.

Advantages

Disadvantages

Access data from anywhere in the world

You need to have an internet connection in order to access your files

You can easily share data with people in other locations

Some people are concerned that data in the cloud may be hacked into

The provider backs up your data and this is often on multiple servers in multiple locations making it more secure

Knowledge check

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