INFO 1 Storage Devices

Data is normally stored as a named file on whatever storage device is to be used.Exactly how the data is stored depends on a number of factors such as:

• Capacity - the amount of data to be stored / size of the file
• Speed - how quickly the user needs to be able to access the data from the storage device
• Portability - whether the data needs to be moved from one place to another such as from home to school
• Durability - How long the storage media can reasonably be expected to last.
• Reliability - whether the data can always be accessed reliably and in the format in which it was saved.

The device that actually holds the data is known as the storage medium (‘media’ is the plural).

The device that saves data onto the storage medium, or reads data from it, is known as the storage device.

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

Internal storage allows the data and applications to be loaded very rapidly into memory, ready for use. The data can be accessed much faster than data which is stored on an external storage device. This is because internal storage devices are connected directly to the motherboard and its data bus whereas external devices are connected through a hardware interface such as USB, which means they are considerably slower to access.

Internal storage also means that if the computer is moved around, it will still retain its most commonly used data.

The main disadvantage of internal storage is that when the hard disk fails, all the data and applications may be lost.

This can be avoided to some extent by using more than one hard disk within the machine. Each hard disk has a copy of all the data, so if one fails the other can carry on. This is called a RAID array. An alternative is to use external drives for backup

External Storage Devices

This takes the form of a stand-alone device that is separate from the computer. External drives are connected to the computer with a cable plugged into a suitable interface such as an USB port. Data then passes back and forth across the interface.

The main advantage of external drives is that they are portable and so data is easily moved from one location to another. External drives also allow safe backup of internally stored data.

The main disadvantage compared to an internal drive is data transfer is slower and they also take up space around the computer. Constant plugging in and out can also physically wear out the port over time.

External storage takes many forms, for example:

• Portable hard disks
• Magnetic tape
• Memory stick / flash drive
• Solid state memory cards
• DVD or CDs

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.

The hard disk is the main storage device in your computer. It is a bit like a filing cabinet: all of your data files and applications software are stored on it.

The hard disk contains a number of metal platters which have been coated with a special magnetic material. The data is stored in this magnetic material. Thus, the hard disk is known as a magnetic storage device.

In order to access the data, the platters spin many thousands of times a second and a magnetic read and write head floats just above the surface of the platter.

Type of storage   - Magnetic

Data access        - Direct access (i.e. not serial access like a tape)

Cost of storage   - Hard disks hold a vast amount of data and are relatively cheap, so on a per-byte basis they are the amongst the cheapest form of storage, although magnetic tape is cheaper.

Capacity            - Can be a Terabyte or more

Speed                - An internal hard disk is faster than external storage devices. It is however, slower than internal solid state memory (SSD) which are now appearing in top end computers

Portability          - An internal hard disk is not intended to be portable - it is screwed to the computer chassis as a permanent fixture unless a replacement is needed.An external hard disk used for backing up data is portable as it comes in various pocket-sized or larger cases, but it is larger than a DVD or USB memory stick.

Durability            - Very durable, it can last for years if treated carefully.Can be damaged by being dropped, experience extreme heat or strong magnetic fields.Data can be written to it and deleted from it an infinite amount of times.

Reliability          - As long as it is not damaged, a hard disk is very reliable., The system detects any failed area on the disk surface and prevents data being written in that spot (these are called 'bad sectors')

Magnetic Tape

The amount of work that you do on your computer at home can easily be backed up onto a CD-RW, CDROM, DVD or a memory stick. However, many organisations, such as your school or an office, need to back up large volumes of data each day. CD-RW, DVD-RW or flash memory sticks are unlikely to be large enough for doing this.

Magnetic tapes can store up to one terabyte of uncompressed data - as much as can be stored on a hard disk.Magnetic tape uses 'serial access' to find a piece of data. This means that to find a specific piece of data, the tape reader has to start at the beginning of the tape and continue fast forwarding until it gets to the piece of data that needed.

Advantages

• Probably the cheapest form of storage per megabyte of storage
• Can store large amounts of data - up to 1 Terabyte per tape cartridge
• Can be set up to do the back up overnight or over the weekend

Disadvantages

• Serial access so can be quite slow to access data
• Need a special piece of equipment to record and read the data on the tape
• The data may be corrupted if the tape is placed near a strong magnetic field e.g. a large speaker or magnet

Type of storage  - Magnetic

Data access      - Serial access (unlike the direct access of a hard disk)

Cost of storage  - This is probably the most cost effective method of storing data which is why it is the technology choice for archiving data.

Capacity           - Can be a Terabyte or more

Speed               - The slowest of all of the storage media from which to access data, which is why it is fine for archiving but not for immediate data retrieval.

Portability        - The magnetic tape itself is fairly small and would fit into a pocket or bag. However, in order to be read, an external tape drive is required. Thus, this form of storage is not considered to be very portable.

Durability        - Although data can be saved to and erased from the tape many times, each tape does have a limited life span and eventually the quality of the data storage will decline. However if a tape is only used once for archiving, then it will last many years, typically 15 years. But of course you also need to keep the tape reading equipment that can read back the data for that time as well.Needs to be protected from extremes of heat.

Reliability          - As long as it is not damaged, a magnetic tape is very reliable method of data storage.

Compact disks are known as optical storage devices.

Data is burned onto the surface of the disk using a laser beam in the CD drive. A laser beam is also used to read the data stored on the disk.

A CD can store around 650 Mb of data. A typical MP3 song is about 3Mb and so a CD can hold about 200 songs.

Compact Disks come in two main forms:

CD-ROM - CD Read Only Memory. Data can only be written once i.e. once the pits are burnt on to the surface, that part of the disk cannot be used again for storage although it can be re-played endless times. Most music discs are of this format.

An old-fashioned name for this is CD-WORM meaning Write-Once-Read-Many but it is hardly a common description these days. You can store more than one data session on a single disk. This is called a 'multi-session' option. This simply means the burner allocates another part of the disk to the next 'session'. CDROMs are more popular than CD-RW because they are so cheap - a few pence each.

CD-RW - CD ReWriteable. This means that you can save data to your disk over and over again, but they are more expensive than the standard CDROM and a USB memory stick can hold much more data in any case.

Advantages

• Small and portable
• Very cheap to produce
• Most computers can read CDs. If there is no CD drive, a DVD drive can usually read them
• Fairly fast to access the data - quicker than magnetic tape

Disadvantages

• Fairly fragile, easy to snap or scratch
• Smaller storage capacity than a hard drive or DVD
• Slower to access than the hard disk.

Type of storage - Optical

Data access - Direct access

Cost of storage - CDs cost more per byte of storage than DVDs, hard disks, magnetic tapes or flash memory

Capacity          - 650 Megabytes

Speed                - Slower than a hard disk or flash memory, faster than magnetic tape

Portability          - Fairly portable. Too large to fit into a pocket, but will easily fit inside a bag. Can be carried between home/school/office.

Durability           - Easily scratched by general wear and tear or by not protecting it in a plastic case. Scratches on the disk surface can damage the data being storedNeeds to be protected from extremes of heat.The CD-RW version has a limited number of read/write cycles.

Reliability         - If scratched, the data might not be able to be read.Needs a computer with a CD or DVD drive in order to read the disk.

• DVDs are amongst the most common methods of copying and backing up data at home.
• A DVD is similar to a CD in that it is an optical device and that a laser is used to store the data and read the data.
• A single layer DVD can store about 4.7Gb of data.
• A double layer DVD can hold over 9Gb of data.
• One problem with the DVD is that the different companies which make them have not agreed on a standard format.
• Because of this, you will see various kinds of DVD disks for sale: DVD-R, DVD+R, DVD-RW and DVD+RW. You have to make sure that you buy the right kind of disk to go with your DVD equipment.

Advantages 

• Very large storage capacity 4.7-9 Gb
• Sound and picture quality is excellent, making them ideal for storing films with video and sound.
• DVDs are now mass produced so they are relatively cheap
• DVD drives can read CDs

Disadvantages

• DVDs do not work in CD drives
• There is no single standard of DVD
• They can be easily damaged by breaking or scratching

Type of storage - Optical

Data access - Direct access

Cost of storage - DVDs cost more per byte of storage than hard disks, magnetic tapes or flash memory. They are less expensive than CDs per byte of storage.

Capacity           - 4.7 (single layer) to 9 Gigabytes (double layer)

Speed               - Slower than a hard disk or flash memory, faster than magnetic tape

Portability         - Fairly portable. Too large to fit into a pocket, but will easily fit inside a bag. Can be carried between home/school/office.

Durability          - Easily scratched by general wear and tear or by not protecting it in a plastic case. Scratches on the disk surface can damage the data being stored. Needs to be protected from extremes of heat.Has a limited number of read/write cycles.

Reliability         - If scratched, the data might not be able to be read.Needs a computer with a DVD drive in order to read the disk. Cannot be read by a CD drive.

A Blu-ray disk is an optical storage device similar to CD and DVD technology. It also uses a laser to read and write to the disk, but instead of a red laser beam, as used in DVD and CD, it uses a blue-violet laser beam.This means it can burn smaller pits on the disk and so it has even more storage capacity than a DVD.

A single layer Blu-ray can store 25Gb whilst a double layer Blu-ray can hold up to 50Gb of data.Blu-ray is commonly used to store high definition films (HD).

There are three format in the specification.

• BD-ROM - read-only format for distributing HD films, computer games, software etc
• BD--R one-time recordable format for HD video recording and data storage
• BD-RE rewritable format

There are also plans to make a BD/DVD combination that has Blu-ray and DVD formats on the same disk so it can be played in both DVD and Blu-ray players.

A single layer disk (25GB) can hold about 4 hours of high definition video or about 12 hours of standard definition video. This huge capacity is very useful for storing training courses in video format.

Advantages 

• Very large storage capacity 25-50 Gb
• Sound and picture quality is excellent, making them ideal for storing HD films.
• Blu-ray disks are now mass produced so they are relatively cheap
• A hybrid Blu-ray / DVD disk can be read in both a Blu-ray and DVD player

Disadvantages 

• Blu-ray does not work in DVD or CD drives
• They can be easily damaged by breaking or scratching
• Using a Blu-ray for backup means that up to 50 GB of data is stored on disk, which may then break. That is a lot of data to lose. Perhaps splitting it into several DVD is less risky.

Type of storage - Optical

Data access  - Direct access

Cost of storage - Blu-ray cost more per byte of storage than hard disks or magnetic tapes.

Capacity           - 25 Gb (single layer) to 50Gb (double layer)

Speed               - Slower than a hard disk or flash memory, faster than magnetic tape

Portability         - Fairly portable. Too large to fit into a pocket, but will easily fit inside a bag. Can be carried between home/school/office.

Durability          - Easily scratched by general wear and tear or by not protecting it in a plastic case. Scratches on the disk surface can damage the data being stored. Needs to be protected from extremes of heat. Has a limited number of read/write cycles.

Reliability          - If scratched, the data might not be able to be read.Needs a computer with a Blu-ray drive in order to read the disk. Cannot be read by a CD drive or DVD drive.

Flash memory is a non-volatile, solid state storage device.(i.e. keeps its data without power and has no moving parts). is increasingly being used in a wide range of devices. Some examples include:

• Small, lightweight USB memory sticks, available for many Gigabytes.
• Memory cards for digital cameras.
• Main internal storage for tablet computers (SSD)
• Digital audio players.
• Mobile phones.
• Video game hardware.

Flash memory offers very fast access to data and programs. However, retrieving data from an external flash memory device such as a USB stick would be slower than an internal hard disk because the speed of access is restricted by the USB link or connection.

With the development of tablet computers, flash memory is used as the main form of internal storage because there is no room for a traditional hard disk. These disks are called SSD (solid state storage). In this case, access to the data stored in flash memory would be much faster than from a hard disk because there are no moving parts involved.

Currently, the down side to using flash memory as the main storage method is that it has a limited number of read/write cycles which limits its useful life span.

Type of Storage - Solid state

Data access       - Direct access

Cost of storage - More expensive per byte of storage than hard disk, magnetic tape, CDs and DVDs

Capacity           - 2 gigabytes upwards.

Speed               - As an external storage device, it is slower than an internal hard disk. It is faster than magnetic tape, CDs and DVDs.If it is used as internal storage, it then becomes faster than an internal hard disk.

Portability        - Very portable as it is designed to be fitted into small electronic devices.

Durability   - Very durable. Resistant to pressure, temperature extremes and accidental damage.It has a limited number of read/write cycles which limits its useful lifeThe part of a USB stick which is inserted into the USB port can be snapped off or damaged rendering the device unusable.

Reliability        - Usually very reliable.

This is another form of external storage, but in this case the device is one or more hard disks NAS storagewithin a case that has an additional network card. In effect it acts as an extra drive on the network.

This means any computer on the network can make use of the storage space it provides. It can also be used for backing up data on the network.

The common name for this device is 'Network Attached Storage' or NAS for short.

A NAS in the home can be used as a 'media server' where films and music are streamed over the network or even connected to the television by a WiFi link.

Storing your data online is becoming an increasingly popular method of data storage.

You pay a monthly fee to an online data storage company and in exchange you are given a certain amount of storage space where you can upload your documents and files. If you have a lot of files and fill up your allocated space, you can simply purchase additional online storage.

Storing data online is secure. You are given a user name and password so that only you or people you authorise can access your files. You can also add extra security by providing users with a password token which generates a password that can only be used once before it expires.

Why do people want to store data online? Here are some reasons:

• It provides an additional way to automatically back up data - a program can be run in the background that uploads a copy of any new file either as it is saved to disk or at a certain time of day.
• It provides peace of mind. Even if you make backups at home or in the office, were there to be a fire or theft, you could lose your original data and also your backups. Having a copy of your data stored online means that you can quickly recover everything.
• You can access your files from anywhere in the world so long as you have an internet connection
• You can share files with other people if you are prepared to give them access to your account