Secondary Storage
Last updated
Last updated
A computer’s primary storage is memory such as RAM and ROM. Secondary storage is non-volatile used to store files and applications, includes hard disk drives (HDDs), solid-state drives (SSDs) and optical disks.
Imagine a hard disk drive (HDD) like a record player for your computer. It has several smooth, round discs coated in a special magnetic material, kind of like tiny magnets all stuck together. These discs spin really fast.
Instead of a needle, the HDD has a little arm with a read/write head. This head hovers close to the discs, but doesn't touch them. It can read the information stored on the magnetic discs or even write new information by changing the magnetism.
A hard disk drive consists of a number of circular platters which are made from a magnetic material. Above each platter hovers an actuating arm on which is a read/write head.
The actuating arm allows the read/write head to access all portions of each platter.
This head can change the magnetic polarity of parts of the platter, which is how data is written. This data is written in concentric tracks, each of which is further divided into sectors.
The platter rotates thousands of times per minute, allowing for good read and write speeds. Hard disk drives typically come in capacities of between 500GB and 5TB.
Adding more platters and decreasing the width of tracks are two ways in which hard disk drives can be kept the same size while increasing their capacity.
Because of the number of moving parts in hard disk drives, they are susceptible to damage from movement. This makes them unsuitable for use in portable devices like phones and tablets.
SSDs are made out of NAND flash memory cells and a controller, which manages the structure of the data that's in the drive.
NAND flash memory is non non-volatile, this means that SSD's content can be retained even when there is no power supply. The memory cells are made up of floating gate transistors which store data by trapping electrical charges.
Data in SSDs is stored as pages, which are combined to make blocks.
Unlike hard disk drives, SSDs are not capable of overwriting data. Instead, an SSD’s controller must completely erase the entirety of a page before writing new information to it.
Because SSDs don’t have any moving parts, they are capable of far higher read and write speeds than HDDs and are suitable for use in portable devices like phones and tablets.
Optical disks include CDs, DVDs and Blu-rays. They store information which can be read optically by a laser. Optical disks can be either read-only, recordable or rewritable depending on what they are to be used for.
Different types of optical disk vary slightly but all follow the same basic principles of operation.
The image below shows a microscope view of the surface of a read-only optical disk. The stripes in the image are called pits, and the areas surrounding them called lands. Pits are burnt into the disk by a high-power laser which permanently deforms the surface.
Unlike hard-disk drives which use tracks and sectors, optical disks have just one continuous track which spirals from the centre of the disk to the outside edge.
When a low-power laser beam is passed over the flat surface of an optical disk, it reflects back onto a photodiode. However, when the laser is incident on a pit, the light from the beam is scattered in different directions rather than reflected back at the photodiode. The resulting pattern of reflections and scatters can be converted into a digital signal of binary 1s and 0s.
On recordable and rewritable optical disks, a pattern of reflections and scatters is created not by pits and lands but by an opaque dye on the disk’s surface. Where there is no dye, the disk reader’s laser beam is reflected off of the optical disk’s surface. Where there is dye, the laser beam is absorbed by the dye and not reflected at all.
Recordable optical disks use a special photosensitive dye which changes from opaque to transparent under a high-power laser which is used to write information to the disk. The dye remains unaffected by the low-power laser used to read the disk.
Typical Capacity
High: 500GB - 5TB
Relatively Low: under 1TB
Very Low: Blu-rays have the highest capacity at 25GB
Read/Write Speeds
Good ≈ 100MB/s
Very high ≈ 500MB/s
Relatively low ≈ 30MB/s
Latency
High
Very Low
High
Portability
Bulky, heavy and easily damaged by movement.
Lightweight and rarely damaged by movement.
Very small and lightweight, can be damaged by scratches and dirt.
Power Consumption
High
Low
High
Suitability
Good for desktop PCs and servers.
Good for laptops, phones and tablets.
Good for sharing and distributing small volumes of data.
