vinodsrivastava.wordpress.com SECTION 3 STORAGE SYSTEM This Section you will learn about: BACK-UP STORAGE NEED OF BACK-UP ACCESSING DATA FROM STORAGE TYPE OF BACKUP(Secondary Storage) Devices o Magnetic- Hard-disk, Floppy Disk, Magnetic Tape o Optical-CD/DVD/CD-R/DVD-R/CD-RW/DVD-RW/Blue-Ray, DVD-RAM o Solid State-USB/Pen Drive, Flash Card What is backing up of data? Backing up refers to the copying of files and data to a different medium (disk, tape, flash drive, etc.) in case of a problem with the main storage device. Backing up files and data on a regular basis is seen as good computing practice and many computer systems can be set to back up files automatically on a regular basis. Why backup? There are various reasons why backups are made. Some of the more common reasons are considered below: Data could be lost due to failure of the original storage device. This could be due to hardware failure (e.g. head crash on a hard drive unit), problems caused by files being overwritten accidentally (or otherwise) or possible corruption of files (e.g. caused by power surges). Hackers could be responsible for the corruption or even loss of data. This may not be their intention (they may only want to gain access to the information for other purposes, e.g. to find personal information such as bank account details). However, the very act of hacking into files could cause problems such as corruption or data loss. Backups are also made in case the files need to be used elsewhere. The original files are then protected against possible corruption or loss..
Types of access The way data is stored and read by different backing storage devices varies considerably. This section briefly describes the two main methods of accessing data. Serial access means going through everything one by one, starting at the beginning, until you reach the item you want. Serial (or Sequential ) access storage device This only allows the data to be read into the computer in the same order in which it was saved. Magnetic tape is an example of sequential access. To find a particular file you need to fast forward through the tape to get to where the file is saved. Direct access means going straight to the item you want without having to go through other ones. If you wanted to print all the names in a database, using serial access would be fine because you are starting at the first and going right through to the end. 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 video tape 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. A computer uses two types of storage: - A main store, consisting of ROM and RAM, and - Backing stores, which can be internal (a hard disk) or external memory stick Main store, or computer memory, is divided into ROM or Read Only Memory and RAM or Random Access Memory. ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. For example, ROM stores the instructions for the computer to start up when it is turned on again. The operating system is loaded from the hard disk and stored in RAM whilst the machine is being used. RAM is a fast temporary type of memory in which programs and data are stored whilst the computer is switched on. For example, when you load a word processing program it is loaded into RAM. The contents of the computer's screen is also held in RAM. If the computer loses power, data stored in RAM is lost. IGCSE ICT-0417@ VKS-Learning Hub Page 2
STORAGE DEVICE Backing Storage Storage holds data, instructions and information for future use. Every computer uses storage to hold system software and application software. Backing storage devices are either internal or external (i.e. plug-in devices) to the computer, and are one of three types: magnetic optical solid state. Magnetic Storage Devices: Magnetic storage (or magnetic recording) is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads. for example:- 1. Hard disk 2. Floppy Disk 3. Magnetic tape HARD DISK Hard Disks are the main internal backing store usually found inside computers. A hard disk contains one or more inflexible, circular platters that use magnetic particles to store data, instructions,and information IGCSE ICT-0417@ VKS-Learning Hub Page 3
SECTION. 3: STORAGE SYSTEM HOW HARD DISK WORKS. They have a very fast data transfer rate and fast access times to data. They have very large memory capacities. They can be fairly easily damaged (e.g. if the correct shut-down procedure on a laptop computer has not been correctly carried out and the computer is then moved). They lack portability unless a portable hard disk drive is used Typical use /Application Used to store operating systems, software and working data. Any application which requires very fast access to data for both reading and writing to. Not for applications which need portability. Used for on-line and real time processes requiring direct access. IGCSE ICT-0417@ VKS-Learning Hub Page 4
Portable hard discs Works in a similar way to fixed hard discs but are connected to the computer externally via a Universal Serial Bus (USB) port. They come with a USB Cable to allow for easy attachment to other computers which allows for easy backing up / sharing of files and data between 2 or more people. Typical storage spaces are 250 gigabytes up to 1 terabyte (1,024 gigabytes). Portable hard disks can be used as back-up systems to prevent loss of data. They can be used to transfer data, files and software between computers. The data access time and data transfer rate is very fast. They have large memory capacities. They can be used as a method of transferring information between computers. As with fixed drives, a portable hard disk can be easily damaged if the user accidentally drops it or does not shut it down correctly after use. Floppy disk drives Floppy disks are still used on some computer systems. They consist of a thin disk of plastic which is housed in a plastic case with a window where the disk can be accessed. As the disk rotates, a read/write head is used to add or read data stored on the surface. They are still used where small files need to be transferred/stored Some older computer systems still make use of this method of storage. Using a CD to store a small file (e.g. a word-processed document) is often regarded as wasteful especially if CD-R is used. Inexpensive and low cost Floppy disks have a very low memory capacity when compared to CD/DVDs, Very few modern computers have floppy disk drives. The data transfer rate is slow compared to more modern data storage devices. Floppy disks are not very robust. IGCSE ICT-0417@ VKS-Learning Hub Page 5
Zip Drive or Superdisks Are very similar to floppy disks. Again they are plastic discs coated with magnetic material. The difference between them is that zip disks can store much more. The one shown stores 100MB and you can get them up to 250MB. Like Floppy disks, zip disks need a specialised zip drive to read and write to the disk. Magnetic tapes A magnetic tape is a very thin strip of plastic which is coated in a magnetic layer.they are read and written to by a read/write head. The data is stored in magnetic areas which represent 1s and 0s. Data is written to and read from the tape in sequence (i.e. in order) for example, if five records A, B, C, D and E were stored they would be in the order E D C B A on the tape; so if record B was to be read it would be necessary to read E, D and C first before getting to the required record. This is known as serial access. This type of storage is useless in a real-time or online application (due to the very slow access speeds) and is best suited to offline or batch processing. Magnetic tapes are used in applications where batch processing is used, for example in clearing bank cheques, utility billing (gas, electricity, water) and producing pay slips. In these applications, there is no need for any specific processing order and speed of data access is not important). They are used as a back-up media since all the data needs to be stored. They are generally less expensive than the equivalent-capacity hard disk. It is a very robust technology. The data transfer rate is fast. serial access so can be quite slow to access data need a special piece of equipment to record and read the data on the tape IGCSE ICT-0417@ VKS-Learning Hub Page 6
Optical storage An optical storage drive that is capable of writing data to a disc uses a laser beam to burn pits (or bumps) into a special material. This data stream is placed in a spiral path, Data is retrieved by a laser beam that reflects off the disc, detecting the pits. Optical drives come in various speeds, spinning between 200 and 4,000 RPMs. Read Only Optical Storage: Which can read but data can not be written over it. Examples Compact Disks (CD) Digital Versatile Disk (DVD) Blue Ray Disk CD-ROMS are used by manufacturers to store smaller files (up to 800MB) such as: Music CD s, Electronic Books etc/ Software such as Microsoft Word etc Advantage More Capacity than Floppy Disk Faster Access than Floppy Disk less expensive than D V D / hard disk drive systems Disadvantage Expensive. than Floppy The data transfer rate and data access time are slower than for hard disks. DVD-ROMS have much larger storage capacities (up to 4.7 GB) than CD-ROMS and are used to store bigger files such as: Movies Larger games such as COD Advantage More Capacity than CD/Floppy Disk Faster Access than Floppy /CD less expensive than hard disk drive systems Disadvantage Expensive. than CD and Floppy DVDs do not work in CD drives The data transfer rate and data access time are slower than for hard disks Blue Ray Disk Largest capacity of all the optical media. They can store up to 100Gb of data. Work in a similar way to DVD ROMS but the laser used to read the data is Blue rather than Red (Red lasers used to read the other disc types). : Used to store High Definition Video (Like high quality movies). Used in some Home Video Consoles (Like Playstation 3) Used to back up Hard Disk Drives in PC s. They have a very large storage capacity, and so are ideal for storing high definition films. The data transfer rate is very fast. The data access speed is also greater than with other optical media. IGCSE ICT-0417@ VKS-Learning Hub Page 7
The disks are relatively expensive. At the time of writing, blu-ray systems still have encryption problems (which are used to stop piracy) when used to store video. Recordable Only Optical Storage Data can be added to the disc but NOT erased. Thin layer of metallic dye is used to record the data onto.when CD-R s and DVD-R s are burnt, the laser makes permanent marks (dots of data which represent 1 s and 0 s) onto the metallic dye. These marks (1 s and 0 s) cannot be erased (Which is why what you add to the disk is permanent). The letter R means that the disc is recordable once only. CD-R and DVD-R They are used for home recordings of music (CD-Rs) and films (DVD-Rs). They are used to store data to be kept for later use or transferred to another computer. They are used in applications where it is necessary to prevent the deletion or overwriting of important data). CD-Rs and DVD-Rs are cheaper than RW disks. Once burned (and finalised), they are like ROM disks. They can only be recorded once, so if an error occurs then the disk become unusable Not all CD/DVD players can read CD-R/DVD-R. CD-RW and DVD-RW The RW means that these disks are a re-writable media and can be written over several times. Unlike CD-R/DVD-R, they don t become ROMs. The recording layer uses a special phase-changing metal alloy. The alloy can switch between crystalline and amorphous (non-crystalline) phases, thus changing its reflectivity to light, depending on the laser beam power. Spots are produced which can be read by a laser and then interpreted as 1s and 0s. The system allows data to be written, erased and re-written many times. CD-RWs and DVD-RWs are used to record radio and television programmes, but can be recorded over time and time again. They are used in closed circuit television (CCTV) systems. CD-RWs and DVD-RWs can be re-used many times. They can use different file formats each time they are used. CD-RWs and DVD-RWs can be relatively expensive media. It is possible to accidentally overwrite data. IGCSE ICT-0417@ VKS-Learning Hub Page 8
SOLID STATE BACKING STORAGE Solid State Drives have no moving parts.they store data as 1 s and 0 s (Just like Magnetic and Optical storage devices) within millions of mini transistors instead of on films of magnetic substance. Memory sticks/pen drives A memory stick/pen drive is a flash memory storage device that plugs in a USB port on a computer or mobile device. Memory sticks and pen drives are used for transporting files between computers or as a back-up store. They are used as a security device a dongle to prevent software piracy. 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 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 Flash memory cards These are a form of electrically erasable programmable read only memory (EEPROM) and are another example of solid state memories. Flash memory cards are used to store photos on digital cameras. Mobile phones use them as memory cards. They are used in MP3 players to store music files. They are used as a back-up store in handheld computer devices. Flash memory cards are very compact, so they can be easily removed and used in another device or used for transferring photos directly to a computer or printer. They are very robust. They are expensive per Gbyte of memory when compared to hard drive disks. They have a finite life in terms of the number of times they can be read from or written to. They have a lower storage capacity than hard disks. IGCSE ICT-0417@ VKS-Learning Hub Page 9
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