Transcript
GCSE Computing A451
Unit 2.5 – Secondary Storage
www.mrfraser.org
Unit 2.5 – Computing Hardware – Secondary Storage Candidates should be able to: a. Explain the need for secondary storage b. Describe common storage technologies such as optical, magnetic and solid state c. Select suitable storage devices and storage media for a given application and justify your choice using characteristics such as capacity, speed, portability, durability and reliability.
Why is secondary storage needed? Secondary storage is data storage that retains its contents when the computer is switched off. It can be used to hold both programs and data. When you run a program or load a file they are copied from secondary storage into the internal memory. When you save a file it is copied from the internal memory to the secondary storage. Secondary Storage compared to Memory: · · ·
It is always slower to access data from secondary storage than from memory. Data stored in secondary storage is permanent so it is NOT lost when the computer is turned off. Data stored in internal memory is lost when the computer is turned off.
What are the common storage technologies? Secondary storage can be divided into three main categories: Magnetic - stores binary data on a disk or tape coated with a material where different areas on the surface can be magnetised North or South to store the binary digits 0 or 1. Optical - stores binary data in a surface where different areas either reflect or do not reflect laser light differently to store the binary digits 0 or 1. Solid state - stores binary data on semi-conductor material where different areas act as switches, either open or closed, to store the binary digits 0 or 1.
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GCSE Computing A451
Unit 2.5 – Secondary Storage
www.mrfraser.org
Storage Devices & Media - Which Applications? Magnetic media (e.g. Hard Drives): ·
Information: Data is stored by magnetising the surface of flat, circular plates called platters. These rotate at very high speed and a read/write head moves from side to side to load and save data. The drive is inside a sealed unit because even a speck of dust could cause the heads to crash. Data is stored on the disc in blocks arranged in tracks and sectors. These are created when the hard disc is first formatted and this must take place before the disc can be used. For a drive to read data from a disc, the read/write head must move in or out to align with the correct track (the time to do this is called the seek time). It must wait then until the correct sector rotates round until it underneath the read/write head.
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Applications: The hard drive is usually the main backing storage media for a typical computer or server. It is used to store: o The operating system (e.g. Microsoft® Windows) o Applications software (e.g. word-processor, database, spreadsheet, etc.) o Files such as documents, music, video etc. A normal home/school microcomputer would have a hard drive with a capacity of over 300 gigabytes.
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Advantages: Very fast access to data. Data can be read directly from any part of the hard disc (random access). The access speed is over 1000 KB per second. Disadvantages: None really! It can however be a real disaster when they eventually fail because few home users have the data on their home computer hard drive backed up.
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Magnetic media - Floppy discs: ·
Information: Floppy disk drives can still be found on many microcomputers and the 3.5 inch floppy media is portable. High density discs for a PC are formatted hold 1.44 MB of data (enough to store about 350 pages of A4 text). A floppy disc needs to be formatted before it can be used but most discs are now sold already formatted for PC's.
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Typical applications: Floppy discs are removable media so are useful for transferring data between computers and for keeping a back-up of small files. 2
GCSE Computing A451
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Unit 2.5 – Secondary Storage
www.mrfraser.org
Advantages: They are very cheap to buy and floppy disc drives are still quite common. Disadvantages: They have very small storage capacity compared to modern alternatives such as USB memory sticks. They are easily physically damaged if unprotected and magnetic fields can damage the data. They are relatively slow to access because floppy discs rotate far more slowly than hard discs, at only six revolutions per second, and only start spinning when requested. The access speed is about 36 KB per second. Not all modern computers have floppy disk drives.
Magnetic media - Magnetic Tape: ·
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Information: Just like the tape in a tape-recorder, the data is written to or read from the tape as it passes the magnetic heads. It is necessary to start at the beginning of the tape and search for the data as the tape goes past the heads (serial access). Typical applications: Magnetic tapes are often used to make a copy of hard discs for back-up reasons. This is automatically done overnight on the KLB network and the tapes are kept in a safe place away from the server. Advantages: Magnetic tape is relatively cheap and tape cassettes can store very large quantities of data (typically 26 GB). Disadvantages: Accessing data is very slow because of the serial access because you cannot go directly to an item of data on the tape as you can with a disc.
Optical Media - CD-Rom: ·
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Information: CD-ROM - means Compact Disc - Read Only Memory. This means you can only read from the media, not write or store data onto it. Like a floppy disc, a CD-ROM is removable media and it only starts spinning when requested so has to spin up to the correct speed each time it is accessed. It is much faster to access than a floppy but it is a lot slower than a hard disc. Typical applications: Most software programs are now sold on CD-Rom. Advantages: CD-ROM's hold large quantities of data (650 MB) and CD drives are common in all modern PCs. They are relatively tough as long as the surface does not get too scratched. Disadvantages: You cannot save files to a CD-Rom (although CD-R and CD-RW discs now exist which can be written to) 3
GCSE Computing A451
Unit 2.5 – Secondary Storage
www.mrfraser.org
Solid State – Solid State Drives: Information: A solid-state drive (SSD) is a data storage device that uses solid-state memory to store data in the same way as a traditional hard disk drive. SSDs use microchips to store data instead of magnetised disks and do not contain any moving parts. SSDs use the same interface as hard disk drives, thus easily replacing them in most applications. Typical applications: Same as hard disk drives. Persistent storage for Operating System. Advantages: Less susceptible to physical shock, no noise, lower access time and latency than hard disk drives. Not affected by magnets! Lower power consumption. Disadvantages: Currently, storage capacity not as high as for HDDs (but this is likely to change soon). Data cannot be overwritten – existing data must be deleted first. Limited number of writes in lifetime of SDD (unless based on DRAM).
Solid State - USB Flash Memory: Information: A USB flash drive consists of a flash memory data storage device integrated with a USB (Universal Serial Bus) interface. USB flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Storage capacities in 2010 were up to approximately 256GB, but these sizes increase rapidly! Typical applications: Same as floppy disks / CD-ROMs were used... Transfer of personal data, secure data storage, used by System Admin to install configuration information / software on networked machines, application carriers, boot operating system, audio players, data backup etc... Advantages: Small, portable, high memory capacity, silent, reliable & durable (no moving parts), little power required, no device drivers required to read/write, Disadvantages: Limited number of write and erase cycles before the drive fails, easily lost!
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