Fundamentals Of Hardware And Software - Secondary Storage

Fundamentals of Hardware and Software SECONDARY STORAGE DEVICES/BACKING STORAGE/AUXILIARY STORAGE This is the storing of information for repeated or for future use. Secondary Storage refers also to long-term storage of processed data. Secondary Storage is Non-Volatile. Secondary Storage is slower than Primary Memory where it takes a longer time to locate information stored in Secondary Storage when compared to Primary Storage and information is written to Secondary Storage slower than Primary Memory Storage. Why Secondary Storage? Devices of memory storage (RAM, ROM, BUFFERS and REGISTERS) are limited in the amount of information and programs instructions they can store and these memory storage devices (except ROM) stores information and program instructions temporarily. Therefore, they are considered to be volatile (except ROM). Secondary Storage allows the storing of information permanently (for a long time). Therefore, Secondary Storage provides non-volatile storage. Secondary Storage Devices (Medias) Secondary Storage Devices are devices that stores and retains information for future or repeated use. Secondary Storage Devices may also be described as the physical components on which information and programs are stored for a long period of time. Examples of Secondary Storage Devices are Magnetic Disks (Magnetic Hard Disk and Magnetic Floppy Disks), Magnetic Tapes and Optical (Lazer) Disks (Compact Disk Read Only Memory CD-ROM, Re-Writable Optical Lazer Disks (CD-RW disk), Write Once Read Many Optical Lazer Disks WORM Disks, Compact Disk Recordable (CD-R disk), Digital Video Disk (DVD Disk) and Magneto Optical Lazer Disks - MO Disks). TYPES OF SECONDARY STORAGE DEVICES MAGNETIC DISKS Magnetic Disk Technology permits direct and immediate access to information and programs stored. The computer can move directly to a specific record or piece of information on the disk instead of having to read through every record one by one until the desired information is located. For this reason, Magnetic Disk is an example of Direct Access Storage Devices (DASD). Magnetic Disks are of two (2) types. They are Magnetic Hard Disks and Magnetic Floppy Disks. MAGNETIC HARD DISKS This is a rigid metal (coated with iron oxide) placed into the computer by the manufacturer used for storing information and programs magnetically. Its rigid (hard) construction allows for higher storage densities. It is used when an organization requires a greater amount of on-line storage (storage area directly accessible by the computer) which Magnetic Floppy Disks and Magnetic Tapes cannot provide. Magnetic Hard Disks are of three (3) types. They are Fixed Disks (Winchester Disks), Removable (Interchangeable, Exchangeable) Disks and the Moving Head Hard disks. TYPES OF MAGNETIC HARD DISKS Interchangeable (Exchangeable/Removable) Magnetic Hard Disks These are Magnetic Hard Disks that are stored off-line (not directly accessible by the computer) and they are loaded or placed into the computer as they are needed to store information or when information stored on them needs to be obtained or retrieved. These disks therefore may be removed from the computer (System Unit) and replaced with another when one disk becomes full of information and may not be able to store any more information or when information is needed from another Hard Disk. Fixed Hard (Winchester Disks) These Magnetic Hard Disks are also made from a hard metal usually made of aluminum with a surface coated with magnetized elements such as Iron Oxide. The manufacturer places this Hard Disk permanently into the computer so that the user may not remove it. Therefore, if the Fixed Hard Disks become filled, it may not be replaced with another Magnetic Hard Disk. If the user needs additional Hard Disk storage, they will have to either erase some information stored on that Fixed Hard Disk or install another hard disk in the computer. A Hard Disk could also be attached outside of the computer at the back of System Unit as a Peripheral Device. Moving Head Magnetic Hard Disks Advantages of Magnetic Hard Disks 1. Individual information may be accessed easily because such information stored has a specific address (in the computer's memory). 2. Provides direct access to information stored. 3. Made of a hard metal, which is not easily damaged which protects the Disk. 4. Allows information to be stored in a well-organized manner. 5. Able to store a large number of software on the computer. Disadvantages of Magnetic Hard Disks 1. Very expensive. 2. Must be kept in a cool and dust free environment. 3. Catches viruses easily (when infected Magnetic Floppy Disks are used in a machine, the infected Magnetic Floppy Disk will infect the Magnetic Hard Disk). 4. This disk can crash and information damaged if the Read/Write Head touches the surface when reading or writing information to this disk. 5. Cannot be easily moved from place to place especially in the case of the Fixed Magnetic Hard Disk because it is placed into the computer permanently. The Exchangeable Magnetic Hard Disks may be transported and carried from one place to another easily. 6. If water and other liquids get into the machine on this disk it may lead to its malfunction. COST The Magnetic Hard Disks may cost between $4, 000 - $12, 000 Jamaica Dollars depending on the amount of storage capacity available and the material used to make it. Storage Capacity/Density A Magnetic Hard Disk can store as much as 1.2 Gigabyte (GB) to 8 or more Gigabyte (GB) of information. Features of the Magnetic Hard Disks 1. Made of a very hard metal. 2. The metal it is made of is coated with magnetized elements such as iron oxide. 3. Not easily damaged. 4. Used to store a large amount of information and software. USE 1. To store a large amount of information and programs. MAGNETIC FLOPPY DISKS These disks are made of a flexible plastic coated with magnetic material that can store information on one side (single density) or on both sides (double density). Magnetic Floppy Disks are mainly used with Microcomputers. This disk comes in 3 sizes: 3 ½", 5 ¼" and 8". The smallest size of Magnetic Floppy Disks (3 ½“) is called Micro Floppy Disks. These disks like the Magnetic Hard Disks are divided into two parts namely TRACKS and SECTORS. Information is stored on the parts of the Magnetic Disks (Floppy and Hard Disks) in binary pattern in magnetized spots. These disks are also an example of Direct Access Storage Devices (DASDs). Advantages of Magnetic Floppy Disks 1. They are cheap. 2. They are physically smaller in size, which permits them to be carried around easily. 3. They are exchangeable. 4. They are lightweight. 5. They are convenient (a user can store information on it and take the disk elsewhere and complete their work). 6. They are portable. 7. They provide direct access to information stored. Disadvantages of Magnetic Floppy Disks 1. They are flexible and so they are easily damaged. 2. The disk drive in which they must be used is more expensive than the disk itself. 3. Accessing of information stored is slower than the accessing time taken to access information stored on Magnetic Hard Disks. 4. Must be kept in a cool and dust free environment and away from heat. COST Magnetic Floppy disks can cost as much as $40 - $60 Jamaican Dollars. Storage Capacity/Density DISK SIZE STORAGE CAPACITIES 3 1/2" 720 KB - 2.8 MB 5 1/4" 360 KB - 1.2 MB 8" Not Available Generally speaking Magnetic Floppy Disks can store between 1.2/1.44 - 2 MB of information! Figure 1 Table showing storage capacities of the different sizes of Magnetic Floppy Disks Features of Magnetic Floppy Disks 1. They posses a much smaller storage capacity than Magnetic Hard Disks. 2. They are much slower to be access (it takes a longer time to find information stored). 3. They are easy to be transported. 4. They are made from flexible plastic coated with magnetic material. USE 1. To store a small amount of information which may be transported easily. Magnetic Tape Magnetic Tape is a sequential secondary storage device on which information is stored in a sequential manner. Magnetic Tape is made of nylon plastic coated with magnetic materials on which information is stored. Examples of Magnetic Tapes are audiocassettes and videocassettes. Magnetic Tape stores information sequentially. In order to find specific information, the Magnetic Tape has to be searched/read randomly (fast forward or rewind) in order for specific information to be located. Magnetic Tapes are more times used to store music, in that songs are stored in a sequential order, from beginning to end. Companies and individuals to back up information stored on their Magnetic Hard Disk periodically also use it. It is relatively slow to locate specific information and processed data stored on Magnetic Tape. Magnetic Tape is one of the oldest secondary storage devices and one of the cheapest storage devices. Magnetic Tapes may be used in older computer systems, which has a Tape Drive (similar to a Disk Drive used to read information from and write information to the Magnetic Tape). They may also be used when an organization requires a large amount of information to be stored sequential (in a set order) at low cost. Magnetic Tape in the only example of a Sequential Access Storage Device (SASD). Advantages of Magnetic Tape 1. A standard Magnetic Tape can store about 200 - 250 MB of information. 2. Magnetic Tape is cheap. Cost a little more than Magnetic Floppy Disks. 3. Magnetic Tapes are convenient to be used to store information because it is light, small and easy to store on storage racks. 4. Magnetic Tapes are re-usable - what is stored on the Magnetic Tape may be erased and new information may then be stored. 5. Large number of records of information may be stored and processed in a sequential manner. 6. A cheap storage device that can be used to back up a large amount of information. Disadvantages of Magnetic Tape 1. Only can store information sequentially. For one to find specific information, the entire tape has to be searched. 2. Relatively stable storage device (can last a long time but may begin to age, crack after a while). 3. After extensive use, the Magnetic Tape becomes stretched and so information becomes partly damaged. 4. Must be kept in a cool environment because it is made of plastic. COST Magnetic Tapes can cost between $60 - $300 Jamaican Dollars. Storage Capacity/Density Magnetic Tapes can store as much as 200 - 250 MB of information. Features 1. Made of flexible nylon plastic coated with magnetic material such as iron oxide. 2. Examples of Magnetic Tapes are audiocassettes and videocassettes. Uses 1. Used for off-line (not accessible by the computer) storage of large (sequential) data files. Tapes are kept in storage racks outside of the computer and are loaded to the computer Tape Drive when needed to store or when information needs to be retrieved from them. 2. May be used for back-up storage for on-line information stored on Magnetic Hard Disk to ensure that a second copy of information is available in case virus destroys information stored on the Magnetic Hard Disk. 3. To store a large amount on information in sequential order. E.g. Bill payment information belonging to Cable and Wireless Jamaica (CWJ), National Water Commission, Jamaica Public Service, Paymasters, etc. 4. Protection against lost of valuable data. Magnetic Tape is used primarily as a back up medium for Magnetic Disk Storage. 5. Archiving Files. Important files no longer needed for active processing can be archived to Magnetic Tape. For example, banks archive old transactions (cheques, deposit slips, etc.) for a number of years. 6. File Portability between computers. Large amount of information can be transferred between computers by writing to Magnetic Tape at the source site and reading from the Magnetic Tape at the destination site. Optical (Lazer) Disk This is a secondary storage device that uses lazer technology to store information to and read information stored. Optical (Lazer) Disks are made from fiber plastic material. Optical (Lazer) Disks is an alternative storage technology being used now widely in today's computer society over the traditional storage technology of Magnetic Technology commonly used with Magnetic Tape, Magnetic Floppy and Magnetic Hard Disks. Optical (Lazer) disks comes in many types namely Compact Disk Read Only Memory Optical (Lazer) Disk (CD-ROM), Re-Writable Optical (Lazer) Disk (CD-R Disk), Write Once Read Many Optical (Lazer) Disks) - WORM Disks, Digital Video Disk (DVD), Compact Disk Recordable (CD-R Disk), etc. Optical (Lazer) Disk provides the user with Multimedia possibilities. Multimedia is the integration of two or more types or medias such as text, animation, graphics, sound and full motion video into computer based application. This secondary storage device is popularly used where enormous storage space is needed cheaply and in a compact form providing easy retrieval. Optical (Lazer) Disks are examples of Direct Access Storage Devices (DASDs). Compact Disk Read Only Memory (CD-ROM) Optical (Lazer) Disk This is a read only storage device, which means that no new data can be written to, or stored on it by the user. The information stored on this storage device may only be read from and cannot be changed by the user. Unlike Compact Disk Re-Writable Optical Disk, where the user may read and write (change) information stored, with CD-ROM and WORM Optical Disks, the user can only access/read information stored and not change what is stored or add new information to it. CD-ROM disks are widely used to store reference materials where large amount of information such as an encyclopedia may be stored on a single disk. CD-ROM is now widely being used to store images (graphics, animations, drawings, symbols, etc.), in the storing of interactive entertainment (games) and in the storing of educational software where multi-media is now being used to stimulate interest. Re-Writable Optical (Lazer) Disk/Compact Disk Rewritable (CD-RW) These Optical (Lazer) Disks uses a combination of magnetic and optical technologies to read and store information. On this storage device, the user may read information stored from the disk and the user may change or add additional information if necessary. The user may change information stored on these disks. Write Once Read Only (WORM) Optical (Lazer) Disks These disks allow information to be written to once and read from many times. The user may not change information stored on these disks. WORM Optical Disks are used by people to store and retrieve archival information, historical data, proprietary data (such as the text for a book), etc. Because WORM disks do not allow information stored to be erased or changed once the information has been burnt on, authors and publishers of books are now placing their books on Compact Disks rather than distributing them only in printed form. WORM disks are used where security of information is vital, crucial and essential such as for the storing of Legal Documents such as laws and information about countries, information for encyclopedias, financial yearly report, and data which has been compiled for government purposes such as a census, population study, research, etc. Digital Video (Versatile) Disk (DVD Disk) This storage device looks similarly to a regular Compact Disk. The only difference is that the DVD disk is smaller in size. The DVD disk can store as much as 7 – 14 times as much information as a regular Compact Disk. Some can store as much as 10 GB. This disk is used to store full-length video movies. Advantages 1. Provides better quality sound and graphics. 2. Provides the user with the possibility of multi-media applications 3. May be used to store educational programs and encyclopedias with various graphics, availability of sound, pictures, music, etc therefore making learning fun and more interesting. 4. Allows users to store large amount of information in a small space. 5. Not very expensive. 6. Not damaged when water or liquid catches the surface. Disadvantages 1. If the disk surface becomes scratched, information stored becomes partly damaged. 2. These disks must be handled with extreme care and kept in a cool environment - away from excessive heat. 3. More expensive than Magnetic Floppy Disk and Magnetic Tape. 4. Easily damaged due to its glass-like surface. 5. Accessing of information stored on the Optical (Lazer) Disk is slower than the accessing of information stored on Magnetic Hard Disk as the computer is able to spin the Magnetic Hard Disk several times faster than it is able to spin an Optical (Lazer) Disk. COST Optical (Lazer) Disks can cost between $550 - $900 Jamaican Dollars. STORAGE CAPACITY/DENSITY This storage device can store 660 MB of information (as much as 300 High Density Magnetic Floppy Disks). Feature 1. Made of a plastic-like glass material such as fiberglass. 2. The information stored on CD-ROM and WORM disk may only be read and not changed while information stored on Re-Writable Optical (Lazer) Disk may be read from and changed by the user. Use 1. To store a large amount of information in a compact form. 2. To store multimedia applications. TERMS ASSOCIATED WITH BACKING/SECONDARY STORAGE DEVICES Read/Write Head This is a component of the disk/tape drive of the computer that reads and writes information to and from Magnetic Disks/Tapes. Access Arm This is a disk drive mechanism used to position the Read/Write Head over a particular Track on which information is stored on the Magnetic Disk/Tape. Tracks This is the portion of the Magnetic Disk that can be accessed in any given setting by the Read/Write Head. Sector This is a pie-shaped division of each track on which data is stored and may be retrieved subsequently. Cylinders When 10 Tracks comes together they form a Cylinder. Buffer Intermediate Memory that temporarily stores/holds data being taken from Main Memory and being sent to an Output Device (example Printer, Monitor, etc.). Direct Access Data on any sector can be accessed directly by using its disk address. The Read/Write Head is moved until it is over the specified Track (without reading information on the way as it move) and then as the specified Sector of the Track passes beneath the Read/Write Head, information is read from the disk surface into Main Memory. In other words, the ability to directly access information stored on a storage device no matter the physical location stored. Access Time This is the time taken to read information from a secondary storage device into Main Memory by the Read/Write Head. Sequential Access This is when data is accessed sequentially (in a set order as information is stored). Seek Time This is the time taken by the Read/Write Head to reach the Track on which information is stored. Data Transfer Time (Rate) This is the time taken to read information into Main Memory/Primary Storage on a secondary storage device. Rotational Delay This is the time taken to bring a particular section of the disk (Track or Sector) beneath the Read/Write Head for information to be read from or written to it. Direct Access Time This refers to the speed at which information stored in Primary (Main) Memory Storage or Secondary Storage may be located on a secondary storage device. Single Density This is the ability to store information on one side of the Magnetic Floppy Disk. Double Density This is the ability to store information on both sides of the Magnetic Floppy Disk. Disk Density This refers to the number of bits that can be stored per unit area (e.g. per inch) on the disk surface. Additional Content: File Compression Zip Disk/Zip Drive Super Disk Compact Disk Recordable Disk (CD-R) Compact Disk Rewritable (CD-RW) Disk Formatting/Disk Initializing Compact Disk Writers Holographic (Memory) Storage Disk Care: Do’s and Don’ts WORM Disk Partition Directory Files Compact Disk Writers WORM Disk Partition Directory Files File Allocation Table (FAT)