Storage devices and media

What is data storage?

When we talk about ‘storing’ data, we mean putting the data in a known place. We can later come back to that place and get our data back again.

‘Writing’ data or ‘saving’ data are other ways of saying ‘storing’ data.

‘Reading’ data, ‘retrieving’ data or ‘opening’ a file are ways of saying that we are getting our data back from its storage location.

Main memory


Main memory (sometimes known as internal memory or primary storage) is another name for RAM (and ROM).

Main memory is usually used to store data temporarily. In the case of RAM, it is volatile (this means that when power is switched off all of the data in the memory disappears).

Main memory is used to store data whilst it is being processed by the CPU. Data can be put into memory, and read back from it, very quickly.

Backing Storage

Backing storage (sometimes known as secondary storage) is the name for all other data storage devices in a computer: hard-drive, etc.

Backing storage is usually non-volatile, so it is generally used to store data for a long time.

Sometimes the storage medium is a fixed (permanent) part of the storage device, e.g. the magnetic coated discs built into a hard drive

Sometimes the storage medium is removable from the device, e.g. a CD-ROM can be taken out of a CD drive.

Why back up data?

Serial / Sequential Access

A serial (or sequential) access storage device is one that stores files one-by-one in a sequence.

A non-computer serial access device that will be familiar to you is a VHS videotape. Because video is stored on a long piece of tape, when TV shows are recorded onto the tape, they go on one-by-one, in order...

Direct / Random Access

A direct (or ‘random’) access storage device is one that stores files so that they can be instantly accessed - there is no need to search through other files to get to the one you want.

An example of a direct access device would be a DVD movie. Unlike the VHS videotape movie, you can jump to any scene on a DVD.

All parts of the DVD are directly accessible. This type of file storage is called direct access.

Data storage capacity

Data storage capacity is measured in bytes (B).

A thousand bytes is known as a kilobyte (kB)
1,000B = 1kB

A million bytes is known as a megabyte (MB)

1,000,000B = 1MB

A thousand million bytes is called a gigabyte (GB)

1,000,000,000B = 1GB

A million million bytes is called a terabyte (TB)

1,000,000,000,000B = 1TB

Data access speeds

Access speeds are measured in bytes per second (Bps).

Slow devices have speeds measured in thousands of Bps (kBps).

E.g. a floppy disc can save/read data at a speed of 60kBps

Fast devices have speeds measured in millions of Bps (MBps).

E.g. a hard-drive can save/read data at a speed of 300MBps (5000 times quicker than the floppy!)

Different types of storage

Is it magnetic?

Magnetic storage media and devices store data in the form of tiny magnetised dots. These dots are created, read and erased using magnetic fields created by very tiny electromagnets.

In the case of magnetic tape the dots are arranged along the length of a long plastic strip which has been coated with a magnetisable layer (audio and video tapes use a similar technology).

In the case of magnetic discs (e.g. floppy disc or hard-drive), the dots are arranged in circles on the surface of a plastic, metal or glass disc that has a magnetisable coating.

Examples of magnetic storage

Fixed hard disk

Portable hard disk

Floppy hard disk

Magnetic tape


 Fixed hard-drive   (Magnetic)

Hard-drives have a very large storage capacity (up to 16TB). They can be used to store vast amounts of data. Hard-drives are random access devices and can be used to store all types of films, including huge files such as movies. Data access speeds are very fast.

Data is stored inside a hard-drive on rotating metal or glass discs (called ‘platters’).

A hard-drive built into the case of a computer is known as ‘fixed’. Almost every computer has a fixed hard-drive.

Fixed hard-drives act as the main backing storage device for almost all computers since they provide almost instant access to files (random access and high access speeds).

Used for:

Storing Operating system
Storing system software
Storing application software
Storing files (mp3,mp4,avi etc...)
Storing documents
Used in file servers

Portable hard-drives

A portable hard-drive is one that is placed into a small case along with some electronics that allow the hard-drive to be accessed using a USB or similar connection.

Portable hard-drives allow very large amounts of data to be transported from computer to computer.

Uses of portable hard disks
​
Store large files such as video
Used for backing up data
Transferring data between devices via USB connection

Floppy disk

A removable, portable, cheap, low-capacity (1.44MB) storage medium.

​Floppy discs are random access devices used for transfer small amounts of data between computers, or to back-up small files, etc. Access times are slow.

Almost every PC used to have a floppy disc drive. These are obsolete now, having been replaced by higher capacity technology such as CD-ROMs, DVDs and USB memory sticks.

Uses of floppy disks

Transfer small files

The can be write protected so they cant be overwritten

Magnetic tape

Magnetic tape is a large capacity, serial access medium.

Because it is a serial access medium, accessing individual files on a tape is slow.

Tapes are used where large amounts of data need to be stored, but where quick access to individual files is not required. A typical use is for data back-up (lots of data, but rarely only accessed in an emergency)

Tapes are also used and in some batch-processing applications (e.g. to hold the list of data that will be processed).

There are magnetic tapes that will hold hundreds of Tb

Uses of magnetic tapes

If large amounts of data needs backing up
If speed is not very important

Backing up school networks (overnight)

Backing up business networks (overnight)

Optical Storage

Optical storage devices save data as patterns of dots that can be read using light. A laser beam is the usual light source.

The data on the storage medium is read by bouncing the laser beam off the surface of the medium. If the beam hits a dot it is reflected back differently to how it would be if there were no dot. This difference can be detected, so the data can be read.

Dots can be created using the laser beam (for media that is writable such as CD-Rs). The beam is used in a high-power mode to actually mark the surface of the medium, making a dot. This process is known as ‘burning’ data onto a disc.

Read-Only optical disks

CD-ROM
Compact Disc - Read-Only Memory (CD-ROM) discs can hold around 800MB of data. The data cannot be altered (non-volatile), so cannot be accidently deleted. CD-ROMs are random-access devices.

CD-ROMs are used to distribute all sorts of data: software (e.g. office applications or games), music, electronic books (e.g. an encyclopaedia with sound and video.)

​

Uses of CD-ROM's

Music tracks
Software
Audio Books

DVD-ROM
Digital Versatile Disc - Read-Only Memory (DVD-ROM) discs can hold around 4.7GB of data (a dual-layer DVD can hold twice that).

DVD-ROMs are random-access devices.
​
DVD-ROMs are used in the same way as CD-ROMs (see above) but, since they can hold more data, they are also used to store high-quality video.

Uses of DVD ROM's

Movies
Computer games
Visual and sound data

Advantages and disadvantages of CD and DVD ROM's

CD-R & DVD-R

CD-Recordable (CD-R) and DVD-recordable (DVD-R) discs can have data burnt onto them, but not erased.

You can keep adding data until the disc is full, but you cannot remove any data or re-use a full disc.

When CD-Rs and DVD-Rs are burnt, the laser makes permanent marks on the silver-coloured metal layer. This is why these discs cannot be erased.

Uses of CD-R & DVD-R

Creating personalized music cd's
DVD-R can be used to store personalized movies
Back-up data
Transfer data from one device to another

CD-RW & DVD-RW

CD-RW
Stands for "Compact Disc Re-Writable."

A CD-RW is a blank CD that can be written to by a CD burner.

Unlike a CD-R (CD-Recordable), a CD-RW can be written to multiple times. The data burned on a CD-RW cannot be changed, but it can be erased. Therefore, you have to completely erase a CD-RW every time you want to change the files or add new data.

​While it may be somewhat inconvenient, this capability makes CD-RWs a good choice for making frequent backups. However, because CD-RWs can be erased, they don't store data reliably for as long as CD-Rs do. Therefore, you should use regular CD-Rs for long-term backups.

DVD-RW

Stands for "Digital Versatile Disk Rewritable."

A DVD-RW is like a DVD-R but can be erased and written to again.

Like CD-RWs, DVD-RWs must be erased in order for new data to be added. DVD-RWs can hold 4.7GB of data and do not come in double-layered or double-sided versions like DVD-Rs do.

Because of their large capacity and ability to be used mulitple times, DVD-RW discs are a great solution for frequent backups.

To record data onto a DVD-RW disc, you'll need a DVD burner that supports the DVD-RW format.

Advantages & disadvantages of using DVD-RW & CD-RW

Blu-ray Disks

Blu-ray is an optical disc format such as CD and DVD.

It was developed for recording and playing back high-definition (HD) video and for storing large amounts of data.

While a CD can hold 700 MB of data and a basic DVD can hold 4.7 GB of data, a single Blu-ray disc can hold up to 25 GB of data.

Even a double sided, dual layer DVD (which are not common) can only hold 17 GB of data. Dual-layer Blu-ray discs will be able to store 50 GB of data. That is equivalent to 4 hours of HDTV.

Blu-ray discs can hold more information than other optical media because of the blue lasers the drives use. The laser is actually blue-violet, but "Blu-ray" rolls off the tounge a little easier than "Blu-violet-ray."

​The blue-violet laser has a shorter wavelength than the red lasers used for CDs and DVDs (405nm compared to 650nm). This allows the laser to focus on a smaller area, which makes it possible to cram significantly more data on a disc the same size as a CD or DVD. 

DVD RAM

Stands for "Digital Versatile Disc Random Access Memory."

DVD-RAMs are writable DVDs that can be erased and rewritten like DVD-RW and DVD + RW discs. 

Unlike the other two writable DVD formats, DVD-RAM discs support advanced error correction and defect management. While these features slow down the maximum data transfer rate for DVD-RAM discs, it also makes the discs more reliable.

Early DVD-RAM discs required an enclosing cartridge, which meant they would not fit in most DVD players or DVD-ROM drives. Therefore, you would need a DVD-RAM drive to use DVD-RAM discs, as well as burn them. 

Newer DVD-RAM discs, however, can be used without a cartridge. These discs can be played in any DVD player that supports the DVD-RAM format. 

​While the first DVD-RAM media could only hold 2.6GB on a single-sided disc, newer double-sided discs can store up to 9.4GB.

Solid State Storage

Solid state, at its most basic level, means "no moving parts." Therefore, solid state electronic devices are made up of solid components that do not move.

Some examples include computer motherboards and integrated circuits. Devices that use only solid state parts, such as television sets, speakers, and digital watches, are often referred to as solid state products.

Flash memory devices are solid state products, while hard drives are not. This is because hard drives use a spinning disk and moving drive head to read and write data, while flash memory uses electric charges to perform the same functions. For this reason, flash memory devices are seen as more durable than hard drives. This is why flash memory is often used in products such as portable MP3 players and digital cameras.

​ Because solid state devices have no moving parts, they are less likely to break down than devices that have mobile mechanisms. For this reason, it is often more worthwhile to buy an extended warranty on electronics that have moving parts than those that do not. That is something you may want to think about next time you are shopping.



​Advantages & disadvantages of solid state hard-drives



​Advantages & disadvantages of memory sticks/pen-drives



​Advantages & disadvantages of flash memory cards

How to compare different storage devices