Chapter 3 - Storage devices and media

Magnetic media and magnetic storage devices

Magnetic tape drives

• Very thin strip of plastic

• Coated in a magnetic layer (iron oxide)

• read/write head

• Data is stored as a magnetised area(represents a 1) or demagnetised area(represents a 0)

• Vast storage capacity

• Magnetic tapes are still used on large industrial or university computers.

Uses of magnetic tape

• Use in applications - clearing bank cheques, billing(gas, electricity, water), payslips

• Used as a backup media where vast amounts of data need to be stored

• Long-term archiving

• Huge data storage capacities

• Very stable

• Long-term storage

Advantages of magnetic tape

• Less expensive than HDD

• Very robust technology

• Huge data storage capacity

• Data transfer rate is fast

Disadvantages of magnetic tape

• Very slow data access times

• Use serial access

• When updating, another tape is needed to store the final updated version

• Requires use of a master tape(the original tape)

Hard Disk Drives (HDD)

• Most common methods used to store data on a computer

• Data is stored in a digital format on magnetic surfaces of hard disks

• Will very often have more than one platter depending on its capacity

• read/write heads can move very quickly

• 50 /s

• How HDD works:

- Actuators are used to move the read/write heads

- these are similar to the electromagnets used in speakers - hence the name

- platter is rotating at up to 10000 rpm

- read/write head contains a tiny magnet which allows the data on the platter to be read

- platters are made from glass, ceramic or aluminium which are coated in iron oxide

- data is stored in concentric, circular tracks

- known as a file allocation table (FAT)

• Have much faster data access times than magnetic tape

• What is latency?

- Latency is defined as the time it takes for a specific block of data on a data track to rotate around to the read/write head

Uses of fixed hard disk drives

• To store the operating systems software and working data/files

• Used in real-time systems

• Used in file servers for computer networks

Advantages of fixed hard disk drives

• Very fast data transfer rate and fast access times of data

• Very large memory capacities

Disadvantages of fixed hard disk drives

• Easily damaged

• Many moving parts which can affect their overall reliability

• read/write operation can be quite noisy when compared to solid-state drives

Portable hard disk drives

• HDDs external to the computer and can be connected to the computer using one of the USB ports

Uses of portable hard disk drives

• Can be used as backup systems to prevent loss of data

• To transfer data/files/software between computers

Advantages of portable hard disk drives

• Data access time and data transfer rate are very fast

• Have a large memory capacity

• Used as a method of transferring information between computers

Disadvantages of portable hard disk drives

• Be easily damaged if the user accidentally drops it or does not correctly shut down the drive after use

• Data transfer rate is not as fast as for fixed hard drives

Optical media and optical storage devices

CD/DVD optical disks

• Are read from or written to by optical storage devices

• Can be built in to a computer or connected externally via a USB cable

• Optical properties of laser light, which is used to read data and to write data on the surface of the disk

• CDs and DVDs can be designated as:

- ‘R’ (write once only)

- ‘RW’ (can be written to or read from many times) 

- ‘ROM’ (read only)

• Use a thin layer of metal alloy or a light-sensitive organic dye coating to store the data

• Data is stored in pits and lands on the spiral track

• A red laser is used to read and write the data

Uses of CD-R and DVD-R

• Home recording of music (CD-R) and films (DVD-R)

• Used to store data to be kept for later use or to be transferred to another computer

Advantages of CD-R and DVD-R

• Cheaper medium than RW disks

• Once burned (and finalised) they behave like a ROM

Disadvantages of CD-R and DVD-R

• Can be only be recorded once; if an error occurs then the disk has to be thrown away

• Not all CD/DVD players can read CD-R/DVD-R

Uses of CD-RW/DVD-RW

• Used to record television programs which can be recorded over, time and time again

• Used in CCTV systems

• Can be used as a backup device for files and data

Advantages of CD-RW/DVD-RW

• Can be written over many times

• Can use different file formats each time it is used

• Not as wasteful

Disadvantages of CD-RW/DVD-RW

• Can be relatively expensive media

• Possible to accidentally overwrite data

Uses of CD-ROM/DVD-ROM

• Optical disks are read-only memory (ROM) which means they cannot be written over and can only be read

• Are permanent method of data storage

• Used to store music files and to store software, computer games

• DVD-ROM has much larger storage and is used to store films

• CD-ROMs and DVD-ROMs are used in applications where there is a real need to prevent the deletion or overwriting of important data

Advantages of CD-ROM/DVD-ROM

• Less expensive than hard disk drive systems

Disadvantages of CD-ROM/DVD-ROM

• Data transfer rate/data access time is slower than for hard disks

Blu-ray discs

• Different to DVDs in their construction

• Carry out read/write operations

• Differences between DVD and Blu-ray:

- a blue laser, rather than a red laser is used to carry out Blu-ray read and write operations; the wavelength of blue light is only 405 nanometres

- Blu-ray can store up to five times more data than normal DVD

Blu-ray discs use a 1.2 mm thick disk however dual-layer Blu-ray and normal DVDs use 0.6 mm thick disks

- data transfer rate for DVd is 10 Mbps and for Blu-ray disc is 36 Mbps

Comparison of the capacity and interactivity of DVDs and Blu-ray discs

• Single layer DVD has a storage capacity of 4.7 GB

• Single layer Blu-ray disc has a storage capacity of 27 GB

• Dual layer Blu-ray disc has a storage capacity of 50 GB

Advantages of Blu-ray compared to DVD

• Record high definition television programmes

• Skip quickly to any part of the disc

• Create playlist of recorded movies and television programmes

• Edit or re-order programmes

• Automatically search for empty space on the disc to avoid over-recording

• Access websites and download subtitles

Uses of Blu-ray discs

• Home video consoles

• Storing and playing back movies

• Computers can use this technology for data storage or backing up hard drives

• Camcorders can use this media

Advantages of Blu-ray discs

• Very large storage capacity, therefore ideal for storing high definition movies

• Very fast data transfer rate

• Data access speed is also greater than with other optical media

• Secure encryption system

Disadvantages of Blu-ray discs

• Expensive

• Encryption problems when used to store video

Solid-state media and solid-state storage devices

• Latency is a major issue in HDDs

• Solid-state technology removes this issue

• Most common type of solid-state technology stores data by controlling the movement transistors within the chip

Floating gate and control gate transistors

• Floating gate and control gate transistors use CMOS(complementary metal oxide semi-conductor) NAND technology

• Floating gate transistor has a value of 1 when it is charged and a value of 0 when it is not

Solid-state drives (SSD)

Uses of SSDs

• Use solid-state media 

• Can be used in the same way as an HDD

• Have enabled laptop computers to become thinner and much lighter

Advantages of SSDs

• Main benefits of using an SSD rather than HDD are : 

- more reliable

- lighter

- a lower power consumption

- run much cooler than HDDs

- no moving parts, they are very thin

- data transfer speed for SSDs is also much faster than for HDDs

Disadvantages of SSDs

• Most solid-state storage devices are conservatively rated at only 20 GB write operations per day over a three-year period

• SSD technology is still not used in all servers

Pen drives

• Small portable devices that make use of solid-state technology

• Small, lightweight portable devices 

• Suitable method for transferring files between computers

• Small backup devices

• USB flash drives which draw their power from the computer via the USB connection

• Portable media player

• USB flash storage

• Any device that uses solid-state flash drive with a USB connector

• Pen drive is a type of flash drive with a particular purpose

Uses of memory sticks/pen drives

• Transporting files between computers 

• Backing store

• Security device to prevent software piracy

Advantages of memory sticks/pen drives

• Compact and portable media

• Robust

• Does not need additional software

• Not affected by magnetic fields

Disadvantages of memory sticks/pen drives

• Cannot write protect the data/files by making it ‘read-only’

• Easy to lose

• Incorrect removal will corrupt data and make it useless

Memory cards

• Solid-state technology

• Can be inserted into a device which can read the card or allow data to be written to the card

• SD cards (secure digital card)

• XD cards (extreme digital card)

• CFast card (compact fast card)

• The XD card is a type of removable memory card designed for use in digital cameras

• The SD card is a type of very small card with a very high-capacity memory

• The CFast card is a memory card format which was developed to allow solid-state technology to be used in a very small portable device

The future of storage devices

• Using the cloud to store all their files

• By streaming their music from the internet