What is Data Storage? Backing Storage, Study notes of Technology

Download What is Data Storage? Backing Storage and more Study notes Technology in PDF only on Docsity! ICT [0417] Resource Person: Saem M Tariq [Lahore Grammar School] What is Data Storage? When we talk about ‘storing’ data, we mean putting the data in a known place. We can later come back to that place and get our data back again. ‘Writing’ data or ‘saving’ data are other ways of saying ‘storing’ data. ‘Reading’ data, ‘retrieving’ data or ‘opening’ a file are ways of saying that we are getting our data back from its storage location. Backing Storage vs Main Memory Main memory (sometimes known as internal memory or primary storage) is another name for RAM (and ROM). Main memory is usually used to store data temporarily. In the case of RAM, it is volatile (this means that when power is switched off all of the data in the memory disappears). Main memory is used to store data whilst it is being processed by the CPU. Data can be put into memory, and read back from it, very quickly. Memory is fast to access, but only holds data temporarily... Backing Storage Backing storage (sometimes known as secondary storage) is the name for all other data storage devices in a computer: hard-drive, etc. Backing storage is usually non-volatile, so it is generally used to store data for a long time. Backing storage devices are slower to access, but can hold data permanently... Storage Media & Devices 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. ICT [0417] Resource Person: Saem M Tariq [Lahore Grammar School] Accessing Stored Data We refer to a collection of data stored in a computer system as a ‘file’. Files are often organised into ‘folders’. Whenever you click ‘Save’ in an application, burn files to a CD- R, copymusic onto your MP3 player, or drag and drop a file onto memory stick, you are using storage devices - devices that can store and retrieve data. Serial / Sequential Access A serial (or sequential) access storage device is one that stores files one-by-one in a sequence. A non-computer serial access device that will be familiar to you is a VHS videotape. Because video is stored on a long piece of tape, when TV shows are recorded onto the tape, they go on one-by-one, in order... If you want to watch a show that you recorded earlier, you have to rewind / fast- forward through all other shows until you find it. The shows are only accessible in the same order that you recorded them. This type of one-by-one storage and access is called serial access. Systems that store things on tape (video, music, computer data, etc.) are always serial access 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. ICT [0417] Resource Person: Saem M Tariq [Lahore Grammar School] Magnetic Tape Magnetic tape is a large capacity, serial access medium. Because it is a serial access medium, accessing individual files on a tape is slow. Tapes are used where large amounts of data need to be stored, but where quick access to individual files is not required. A typical use is fordata back-up (lots of data, but rarely only accessed in an emergency) Tapes are also used and in some batch-processing applications (e.g. to hold the list of data that will be processed). Removeable Media Magnetic Discs Floppy Disc A removable, portable, cheap, low-capacity (1.44MB) storage medium. Floppy discs are random access devices used for transfer small amounts of data between computers, or to back-up small files, etc. Access times are slow. Almost every PC used to have a floppy disc drive. These are obsoletenow, having been replaced by higher capacity technology such as CD-ROMs, DVDs and USB memory sticks. Zip Disc A removable and portable storage medium, similar in appearance to a floppy disk, but with a much higher capacity (100MB, 250MB or 750MB). Zip discs are random access devices which were used for data back-up or moving large files between computers. Another obsolete storage device, zip discs were a popular replacement for floppy discs for a few years, but they never caught on fully before being superseded by cheaper media like CD-ROMs and CD-Rs Jaz Disc A removable and portable storage medium based on hard-drive technology, with a large capacity (1GB or 2GB). Jaz discs are random access devices which were used for data back-upor moving large files between computers. Discs were expensive to buy and not very reliable. ICT [0417] Resource Person: Saem M Tariq [Lahore Grammar School] Like the Zip disc, this system never really caught on and was superseded by far cheaper and more reliable and cheaper technology. Optical Storage Devices / Media Why 'Optical'? Optical storage devices save data as patterns of dots that can be read using light. A laser beam is the usual light source. The data on the storage medium is read by bouncing the laser beam off the surface of the medium. If the beam hits a dot it is reflected back differently to how it would be if there were no dot. This difference can be detected, so the data can be read. Dots can be created using the laser beam (for media that is writable such as CD-Rs). The beam is used in a high-power mode to actually mark the surface of the medium, making a dot. This process is known as ‘burning’ data onto a disc. This is a magnified view of the dots on the surface of a CD. The different patterns of dots correspond to the data stored on the disc. Read-Only Optical Discs Read-only optical discs have data written onto them when they aremanufactured. This data cannot be changed. CD-ROM Compact Disc - Read-Only Memory (CD-ROM) discs can hold around800MB of data. The data cannot be altered (non-volatile), so cannot be accidently deleted. CD-ROMs are random-access devices. CD-ROMs are used to distribute all sorts of data: software (e.g. office applications or games), music, electronic books (e.g. an encyclopaedia with sound and video.) DVD-ROM Digital Versatile Disc - Read-Only Memory (DVD-ROM) discs can hold around 4.7GB of data (a dual-layer DVD can hold twice that). DVD-ROMs are random-access devices. DVD-ROMs are used in the same way as CD-ROMs (see above) but, since they can hold more data, they are also used to store high-quality video. ICT [0417] Resource Person: Saem M Tariq [Lahore Grammar School] High Capacity Optical Discs Blu-Ray Blu-Ray disks are a recent replacement for DVDs. A Blu-Ray disc can hold25 - 50GB of data (a dual-layer Blu-Ray disc can hold twice that). Blu-Ray discs are random-access devices. Blu-Ray discs are used in the same way as DVD-ROMs (see above) but, since they can hold more data, they are also used to store very high-quality, high-definition (HD)video. The 'Blu' part of Blu-Ray refers to the fact that the laser used to read the disc uses blue light instead of red light. Blue light has a shorter wave-length than red light (used with CDs and DVDs). Using a blue laser allows more data to be placed closer together on a Blu-Ray disc, than on a DVD or CD, so Blu-Ray has a much higher storage capacity than these older discs. HD DVD High-density DVD (HD-DVD) discs can hold around 15GB of data (a dual-layer HD-DVD can hold twice that). HD-DVDs are random-accessdevices. HD-DVD discs are used in the same way as DVD-ROMs (see above) but, since they can hold more data, they are also used to store very high-quality, high- definition (HD) video. The HD-DVD format was launched at the same time as Blu-Ray. For about a year they competed to be the 'next DVD'. For various reasons, Blu-Ray won the fight, and the HD-DVD format has been abandoned. Recordable Optical Discs Recordable optical discs can have data written onto them (‘burnt’) by a computer user using a special disc drive (a disc ‘burner’). CD-R and DVD-R CD-Recordable (CD-R) and DVD-recordable (DVD-R) discs can have data burnt onto them, but not erased. You can keep adding data until the disc is full, but you cannot remove any data or re-use a full disc. CD-RW and DVD-RW CD-ReWritable (CD-RW) and DVD-ReWritable (DVD-RW) discs, unlike CD-Rs and DVD-Rs, can have data burnt onto them and also erased so that the discs can be re-used. When CD-Rs and DVD-Rs are burnt, the laser makes permanent marks on the silver-coloured metal layer. This is why these discs cannot be erased.