Storage Size Magnetic/Optical Use



Storage Size Magnetic/Optical Use

Magnetic Tape 20Gb - 200 Gb Magnetic Backing up the school network overnight

 

Hard Disk 40Gb - 250 Gb Magnetic Storing all of your software applications and documents

 

Floppy Disk 1.44 Mb Magnetic Copying a small homework file to take to school.  Unlikely to contain large images.

 

CD-ROM 650 Mb Optical Storing general software applications.

Copying fairly large files to transfer from home to school.

 

DVD 4.7 Gb (single) 9.7 Gb (double) Optical Storing a large computer game or a movie

 Zip Disk 100 Mb to 250 Mb Magnetic Transferring files between computers.  Need a specialist drive.  This storage media is not so commonly used nowadays.

Flash Memory Stick 32 Mb to 512 Mb Solid state Regularly transferring files between home and school, possibly with graphics.

Magnetic tapes come in two forms:

|[pi|Tape reels - these are fairly large and are usually used to back up data from mainframe computers. |

|c] | |

|[pi|Cassettes or cartridges - these are fairly small in size but able to hold enough data to back up the data held on a |

|c] |personal computer or a small network. |

|[pic] |[pic] |

|A magnetic tape cartridge |A magnetic tape reel |

Because it takes a long time to back up onto magnetic tape, it is usually done at night or over a weekend when the computer network  is not in heavy use.

Main advantages

|[pi|Relatively cheap |

|c] | |

|[pi|Can hold in excess of 20GB of data (it is possible to get devices that will back up over 200 GB of data) |

|c] | |

|[pi|Backup capacity is easily expanded by simply using more tape. |

|c] | |

Main disadvantages

|[pi|Serial access - this means that in order to get to something on the tape, you have to go through everything that comes |

|c] |before it.  Think of it like a video tape.  The program that you want to watch might have been recorded half way through |

| |the tape, but you have to wind forward right from the start to get to it. |

|[pi|Slow - Because of serial access, it is relatively slow to find the data that you need (however in an exam question, don't |

|c] |just say 'slow', compare it to a storage device with faster access. |

|[pi|Specialist hardware is needed to read the tapes.  Most standard PCs do not come with the hardware required to use them. |

|c] | |

|[pi|Not suitable for heavy use - As you may realise from video tapes at home - they are a bit fragile with a tendency to |

|c] |stretch and tangle!  Tape is best suited for back-up purposes. |

Tape is a magnetic storage device

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[pic][pic]  The hard disk drive is the main storage device within a computer. It is where all the applications software and data is kept.  Data stored on a hard disk can be accessed much more quickly than data stored on a floppy disk. 

A Hard disk spins around thousands of times per minute inside its metal casing, which is why it makes that whirring noise. Far less than a hairs breadth above the disk, a magnetic read and write head creates the 1 and 0s on to the circular tracks beneath.

Most hard drives are installed out of the way inside the computer, however you can also purchase external drives that plug into the machine.

Modern Hard drives are measured in gigabytes (GB).  The most expensive computers will have the largest hard drives.  Standard systems come with hard drives between 40Gb and 250Gb

It is also possible to add more than one hard disk inside a computer. In fact this is an excellent idea if you wish to back up your data whilst you work - the main disk holds your data whilst the second disk 'mirrors' it in the background. In that way when one of the disks fail (and they will !), the other is keeping your data safe.

There are two types of hard disks (standards):

IDE (Integrated Device Electronics)

|[pi|Standard interface in PC's |

|c] | |

|[pi|More common than SCSI |

|c] | |

|[pi|Cheaper and slower than SCSI |

|c] | |

IDE is by far the most common standard, because it is 'good enough' for most purposes. However, if you wish for high speed and a more robust \ reliable system then SCSI offers an alternative.

SCSI (Small Computer Systems Interface)

|[pi|Faster - high speed of transfer than IDE |

|c] | |

|[pi|More reliable than IDE |

|c] | |

|[pi|Specialist card required |

|c] | |

|[pi|More expensive than IDE. |

|c] | |

 

Advantages and disadvantages of using hard disks as a storage media

Advantages

|[pi|Large storage capacity |

|c] | |

|[pi|Stores and retrieves data much faster than a floppy disk or CD-ROM |

|c] | |

|[pi|Data is not lost when you switch off the computer |

|c] | |

|[pi|Usually fixed inside the computer so cannot get mislaid. |

|c] | |

|[pi|Cheap on a cost per megabyte compared to other storage media. |

|c] | |

|[pi|Hard disks can be replaced and upgraded as necessary |

|c] | |

|[pi|Can have two hard disks in a machine, one can act as a mirror of the other and create a back up copy. |

|c] | |

Disadvantages

|[pi|Hard disks eventually fail which stops the computer from working. |

|c] | |

|[pi|Regular 'head' crashes can damage the surface of the disk, leading to loss of data in that sector. |

|c] | |

|[pi|The disk is fixed inside the computer and cannot easily be transferred to another |

|c] | |

The 'floppy disk' has been with us since the beginning of the personal computers in the 1980's.  The very earliest ones were 8 inches across! Hence the word 'floppy'.

Then came the 5.25 inch format which was popular for a few years. Finally the 3.5 inch floppy disk was developed which offered a hard plastic case and a sliding metal cover to protect the fragile magnetic sheet inside.

The floppy disk drive enables you to transfer small files between computers and also to make backup copies to protect against lost work.

 It used to be one of the most common storage devices for moving data from one PC to another.  However, as file sizes are becoming larger and other, more reliable storage media become increasingly common, people are using floppy disks less frequently.

A floppy disk is made of a flexible substance called Mylar.  They have a magnetic surface which allows the recording of data.  The disk turns in the drive allowing the read/write head to access the disk.

A standard floppy disk can store up to 1.44 Mb of data which is approximately equivalent to 300 pages of A4 text.  However, graphic images are often very large, so you may well find that if you have used Word Art or a large picture, your work will not fit onto a floppy disk.

All disks must be formatted before data can be written to the disk.  Formatting divides the disk up into sections or sectors onto which data files are stored. Floppy disks are usually sold pre-formatted.

Care should be taken when handling disks, to protect the data.  The surface of the disk should not be touched and they should be kept away from extreme temperatures and magnetic fields such as telephones, televisions and microwaves - otherwise you might find all your data has been wiped!

 Advantages

|[pi|Portable - small and lightweight |

|c] | |

|[pi|Allows random access of data - (unlike tape which is serial) |

|c] | |

|[pi|Can provide a valuable means of backing up data |

|c] | |

|[pi|Inexpensive |

|c] | |

|[pi|Useful for transferring files between computers or home and school. |

|c] | |

|[pi|Private data can be stored securely on a floppy disk so that other users on a network cannot gain access to it. |

|c] | |

|[pi|Security tab to stop data being written over. |

|c] | |

|[pi|Most computers have a floppy drive |

|c] | |

|[pi|Can be written to many times. |

|c] | |

 Disadvantages

|[pi|Not very strong - easy to damage |

|c] | |

|[pi|Data can be erased if the disk comes into contact with a magnetic field |

|c] | |

|[pi|Quite slow to access and retrieve data compared to a hard disk drive, but it is faster than tape. |

|c] | |

|[pi|Can transport viruses from one machine to another |

|c] | |

|[pi|Small storage capacity, especially if graphics need to be saved |

|c] | |

|[pi|New computers are starting to be made without floppy drives |

|c] | |

This is a magnetic storage device

CD-ROM stands for compact disk read only memory. 

Although we tend to talk about 'CD-ROMs', it is important to note that there are three types of CD-ROMs:

CDROM

These disks have been pre-recorded with data. For example

|[pi|Music Compact Disks cannot be over written. |

|c] | |

|[pi|Product Catalogues from suppliers. |

|c] | |

|[pi|Computer games. |

|c] | |

|[pi|Documentation such as technical manuals. |

|c] | |

CD-R

These disks are blank but have been designed to be written onto once only. The 'cd-writer' drive uses a laser to burn tiny pits onto the spinning surface of the CD-R media. Each pit represents a '1' data bit. Once the pit is burnt it cannot be erased.

Sometimes your CD Burning software may allow you to burn a 'multi-session' disk. All this is doing is treating the CD-R as a number of smaller areas or 'sessions'.  Each time you start a new session, the drive simply moves to another blank part of the disk.

CD-RW

Often described as 'CD Read-Write'. This technology allows the same area of the disk to be over-written many times (about a 1000 times).

There are two problems with the CD-RW disks - unlike CD-R some drives have a problem reading CD-RW disks that have been burnt by other manufacturers' drives. Secondly, CD-R disks and now so cheap that it is less hassle to simply use a blank disk!.

In an exam question, it is important that you can distinguish between the different types of CDs.

CD-ROM disks do not store data magnetically like floppy disks and hard disks.  Instead, tiny pits are burnt onto the surface by a laser beam in the CD-ROM drive. 

This is why they are known as optical storage devices. A laser beam also reads the information from the disk. The same technique is used for CD music disks which is why many computers with CD-ROM drives can play audio CDs.

A typical CD-ROM can store approximately 650 megabytes of data which is equivalent to about 450 1.4 MB floppy disks.  The entire contents of a text based encyclopaedia takes up only 25% of one standard CD-ROM.

CDs are useful for

|[pi|Backup |

|c] | |

|[pi|Transferring files that do not fit onto a floppy disk |

|c] | |

|[pi|Providing data that you do not want altered by someone else e.g. – software programmes or technical manuals. |

|c] | |

Advantages

|[pi|Data cannot be erased from CD-ROMs |

|c] | |

|[pi|CD-ROMs are small and portable |

|c] | |

|[pi|Very cheap to produce |

|c] | |

|[pi|CD-ROMs have a much larger storage capacity than floppy disks. |

|c] | |

|[pi|Will usually work in a DVD drive. |

|c] | |

Disadvantages

|[pi|Fairly fragile - easy to break or scratch |

|c] | |

|[pi|Because they are portable they can be lost. |

|c] | |

|[pi|Smaller storage capacity than a hard disk |

|c] | |

|[pi|Slower to access than the hard disk |

|c] | |

 

This is an Optical storage device

DVDs stands for Digital Versatile Disk and is a relatively new technology.  Like CD-ROMs, they are an optical storage device.  

These are becoming increasingly popular, and are expected to replace ordinary compact discs and video tapes in the future.  This is because a DVD disc can store between 5-17 gigabytes of data. 

This amount of storage makes it possible to store complete movies along with special features and multiple language tracks.

Advantages

|[pi|Very large storage capacity |

|c] | |

|[pi|Sound and picture quality is excellent which makes them suitable for video and sound |

|c] | |

|[pi|Increasing availability |

|c] | |

|[pi|Price is dropping for both DVDs and DVD drives. |

|c] | |

|[pi|Does not transmit viruses |

|c] | |

|[pi|DVD players can read CD-ROMs. |

|c] | |

Disadvantages

|[pi|Still a relatively new technology so still expensive compared to CD-ROMS. |

|c] | |

|[pi|DVDs do not work in CD ROM drives. |

|c] | |

 

DVD-RW

This new technology is only just becoming affordable to the general public. It is increasingly common to have a DVD re-writer drive in a new personal computer. There are also stand-alone DVD-RW units that allow you record television programs directly onto disk.

Advantages

|[pi|Can store seven times more data than a CD-RW |

|c] | |

|[pi|Suitable for storing video and television programmes |

|c] | |

Disadvantages

|[pi|There are many 'standards' in use, so you have to make sure you buy the correct disk for your drive. |

|c] | |

|[pi|Relatively expensive media compared to a CD-R, so you have to decide whether your data can be stored on one or two 650 Meg |

|c] |disks. |

 

This is an Optical storage device

The Zip drive is similar to a floppy drive but can store 100 MB of data, at least 70 times more than a floppy.  Some zip disks store as much as 250 MB. 

The Zip disk is slightly thicker than a floppy disk and needs a separate drive.  Zip disks are particularly useful for backing up important data or for moving data easily from one computer to another.  Data is compressed, thereby reducing the size of files that are too large to fit onto a floppy disk. 

Advantage:

|[pi|Stores more than a floppy disk |

|c] | |

|[pi|Portable |

|c] | |

Disadvantage

|[pi|More expensive than floppies |

|c] | |

|[pi|Drives to read the disks are not that common |

|c] | |

Zip disks are not commonly used now as a method of storing data.

This is a magnetic storage device

Another kind of storage device has recently emerged. It is called 'Flash' memory. Because it is so new, we have not yet found an common name for it! Sometimes it is called a 'memory stick', USB memory, Key Memory and others.

What it does is to combine a well tried memory technology called 'Flash' with the convenience of the USB connector.

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Flash is a 'solid state' memory i.e it has no moving parts unlike magnetic storage devices, nor does it make use of lasers - unlike optical drives.

 Instead, it works in a similar way to RAM. The key difference is that data is retained in Flash memory even when the power is switched off.

They are now fairly inexpensive, costing from £20 upwards.

Typical sizes range from 32Mbytes up to 8 GB and beyond.

Flash memory is a sold-state storage device.

It is important that you can distinguish between magnetic and optical storage devices.

Magnetic storage:

|[pi|Magnetic Tape |

|c] | |

|[pi|Hard Disk |

|c] | |

|[pi|Floppy Disk |

|c] | |

|[pi|Zip Disk |

|c] | |

|[pi|Magnetic stripes on the back of bank cards |

|c] | |

Magnetic storage devices store the data on a magnetically coated surface.  They can generally be used many times.  They tend to have a large storage capacity when compared to optical media. 

The main disadvantage is that the data held on these can be damaged if the device is put too close to a strong magnetic field such as a loudspeaker.

 

Optical storage:

|[pi|CD-ROM |

|c] | |

|[pi|DVD |

|c] | |

Optical storage devices are read by a laser beam.  Generally they have a more limited storage capacity when compared to magnetic devices.  However, one advantage is that they are more hard wearing than magnetic devices.

 

You need to be able to distinguish between the terms 'Backup' and 'Archiving'.

Backing up

Backing up your data is where you make a copy of what is on your system.  However, the original data is still left in place.  The back up can be stored somewhere separately and just used in case the original data gets corrupted or deleted, or the hard disk gets damaged/stolen.

The backup can be used to restore your data to an earlier version.  

The key point is that if you make a backup, you are not deleting or moving the original data - it stays in the same place.

 

Archiving

Archiving is where you want a copy of your data/records, but you no longer need to keep it on your system.  Once the data is archived, it is usually removed from the system and the archived copy is kept elsewhere. 

This frees up space on the hard disk or wherever the data was originally stored.

Vital Factors deciding on Storage:

[pic]Portability - how easy is the back up to carry around?

[pic]Specialist hardware required - tape devices and zip disks require specialist hardware

[pic]Cost - the cost of specialist hardware and the cost of the media need to be considered ]

[pic]Speed - How quickly can the back up be made? How quickly can the data be retrieved?

[pic]Ease of use - How easy is the device to use? A flash stick or floppy disk are easy, a second hard drive that needs to be removed might take a little more expertise.

[pic]Compatibility - not all machines can read all media.  It is not much use if you use a magnetic disk to back up your system, but the person who needs to access the data cannot use the tape as they don't have the hardware.

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