Backing Storage

A-Level ICT

Introduction

• Unless you want to lose all of the work you have done on your computer, you need to have a way to store it safely.

• There are various types of storage devices, different devices are suitable for different tasks.

• We will be looking at the main ones which you need to know about.

Introduction

Storage

• No matter what you are working on, whether at home, school or in an office, you will want to save your data for use at a later date. Whilst you have the document, spreadsheet, database open, the work is mostly residing in your computer's Random Access Memory (RAM).  However, as you know, this is volatile, so when you turn your computer off, all of your hard work will simply disappear unless you save it to somewhere more permanent.

• There are many different types of storage devices, some you are familiar with, some you may not have come across before.

• In the exam, you are likely to be presented with a situation, given some details and then asked to recommend a suitable storage device, giving justification for your answer.  You may alternatively be provided with a storage device and asked to suggest a situation where it could be used.

Role of storage devices

• Storage devices are designed to hold your data on a permanent or semi-permanent basis.  They are non-volatile, which means that even when the computer is switched off, the data will still be safe.

• Storage media are classified into two categories:– Media that can be over-written many times. – Media that can only be written onto once.

• Storage devices are sometimes referred to as secondary storage.

Storage Capacity

• Data can be stored either in the 'internal memory' or on a 'storage device'.

• The amount of data and instructions that can be stored is measured in 'bytes'.

• One byte contains 8 bits (short for Binary Digit). This is the smallest unit of data that can be stored. Each 'bit' is represented as a binary number, either 1 or 0.

• A single keyboard character such as the letter A or T takes one byte of storage.

• We normally refer to the capacity of a storage device in terms of Kilobytes (KB), Megabytes (MB), Gigabytes (GB) - or even Terabytes and beyond!

Storage Capacity

Storage sizes

Quantity Information

Bit Smallest unit of data, either a 0 or 1

Byte 8 bits

Kilobyte (Kb) Assumed to be 1,000 bytes. In reality, it is really 1,024 bytes.

Megabyte (Mb) 1,000 kilobytes (1,024 Kb)

Gigabyte (Gb) 1,000 megabytes (1,024 Mb)

Terabyte (Tb) 1,000 Gigabytes

Petabyte (Pb 1,000 Terabytes

Exabyte (Eb) 1,000 Petabytes

Zettabyte (Zb) 1,000 Exabytes

Yottabyte (Yb) 1,000 Zettabytes

Magnetic Tape

• Magnetic tapes come in two forms:– Tape reels - these are fairly large and are

usually used to back up data from mainframe computers.

– Cassettes or cartridges - these are fairly small in size but able to hold enough data to back up the data held on a personal computer or a small network.

• 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.

Advantages of Magnetic Tape

• Relatively cheap

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

• Backup capacity is easily expanded by simply using more tape.

Disadvantages of Magnetic Tape

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

• Slow - Because of serial access, it is relatively slow to find the data that you need (however in an exam question, don't just say 'slow', compare it to a storage device with faster access.

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

• Not suitable for heavy use - As you may realise from video tapes at home - they are a bit fragile with a tendency to stretch and tangle!  Tape is best suited for back-up purposes.

Hard Disk

• 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.

Hard Disk

Hard Disk

• 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.

Types of Hard Disk

• There are two types of hard disks (standards):• IDE (Integrated Device Electronics) Standard interface

in PC's More common than SCSI Cheaper and slower than SCSI

• 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) Faster - high speed of transfer than IDE More reliable than IDE Specialist card required More expensive than IDE.

Advantages of Hard Disks

• Large storage capacity • Stores and retrieves data much faster than a floppy disk

or CD-ROM • Data is not lost when you switch off the computer• Usually fixed inside the computer so cannot get mislaid. • Cheap on a cost per megabyte compared to other

storage media. • Hard disks can be replaced and upgraded as necessary • Can have two hard disks in a machine, one can act as a

mirror of the other and create a back up copy.

Disadvantages of Hard Disks

• Hard disks eventually fail which stops the computer from working.

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

• The disk is fixed inside the computer and cannot easily be transferred to another computer.

Floppy Disk

• 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.

Floppy 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 of Floppy Disks

• Portable - small and lightweight• Allows random access of data - (unlike tape which is

serial) • Can provide a valuable means of backing up data• Inexpensive • Useful for transferring files between computers or home

and school. • Private data can be stored securely on a floppy disk so

that other users on a network cannot gain access to it. • Security tab to stop data being written over. • Most computers have a floppy drive• Can be written to many times.

Disadvantages of Floppy Disks

• Not very strong - easy to damage• Data can be erased if the disk comes into contact with a

magnetic field • Quite slow to access and retrieve data compared to a

hard disk drive, but it is faster than tape. • Can transport viruses from one machine to another • Small storage capacity, especially if graphics need to be

saved • New computers are starting to be made without floppy

drives

CD-ROM

• CD-ROM stands for ccompact ddisk rread oonly mmemory. 

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

CD-ROM

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

• Music Compact Disks cannot be over written. Product Catalogues from suppliers. Computer games. Documentation such as technical manuals.

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!.

Storing Data on 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...

• Backup

• Transferring files that do not fit onto a floppy disk

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

Advantages of CDs

• Data cannot be erased from CD-ROMs

• CD-ROMs are small and portable

• Very cheap to produce

• CD-ROMs have a much larger storage capacity than floppy disks.

• Will usually work in a DVD drive.

Disadvantages of CDs

• Fairly fragile - easy to break or scratch

• Because they are portable they can be lost.

• Smaller storage capacity than a hard disk

• Slower to access than the hard disk

DVDs

• DVDs stands for DDigital VVersatile DDisk 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 and Disadvantages of DVDs

• Advantages– Very large storage capacity – Sound and picture quality is excellent which makes them suitable

for video and sound – Increasing availability – Price is dropping for both DVDs and DVD drives. – Does not transmit viruses – DVD players can read CD-ROMs.

• Disadvantages– Still a relatively new technology so still expensive compared to

CD-ROMS. – DVDs do not work in CD ROM drives.

DVD-RW

• This technology has recently become more 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– Can store seven times more data than a CD-RW– Suitable for storing video and television programmes

• Disadvantages– There are many 'standards' in use, so you have to make sure you

buy the correct disk for your drive. – 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 disks.

Zip Disks

• 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: Stores more than a floppy disk Portable• Disadvantage More expensive than floppies Drives to

read the disks are not that common• Zip disks are not commonly used now as a method of

storing data.

Flash Memory Stick

• Another kind of storage device has recently emerged. It is called 'Flash' memory. Because it is so new, there is not yet a 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.

Flash Memory Stick

• 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 €10 upwards for 1GB.

• Typical sizes range from 512Mbyte to 8GB and beyond.

Advantages of Flash Memory Sticks

• They are more compact and portable than floppy disks or CDs/DVDs.

• They hold more data than a floppy disk and nowadays often more than a CD.

• They are more reliable than a floppy disk because they have no moving parts

• They are being developed with fashionable looking outer casings and are almost becoming a 'fashion accessory' much in the way of a mobile phone.

Disdvantages of Flash Memory Sticks

• At the moment, the cost per megabyte of storage is more expensive than floppy disks, CDs or DVDs.

• They can be easily lost

• The metal part which is inserted into the USB port can be snapped off if they are handled roughly

Other Flash storage

Magnetic vs Optical

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

Magnetic Storage

• Magnetic Tape • Hard Disk • Floppy Disk • Zip Disk • Magnetic stripes on the back of bank cards• 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

• CD-ROM

• DVD (all types)

• 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.

Backing up data

• 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 Data

• 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.

Backing Storage exam tips

• If you are given a question about hardware, be it input, output or storage devices, you will usually be asked to identify a piece of hardware for a given situation and justify why you believe it to be suitable.

• It is probably a good idea to use the following points to help you explain your choice: – Portability - how easy is the back up to carry around? – Specialist hardware required - tape devices and zip disks require specialist

hardware – Cost - the cost of specialist hardware and the cost of the media need to be

considered – Speed - How quickly can the back up be made? How quickly can the data be

retrieved? – 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.

– 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.