Information Technology Storage Devices free essay sample

Storage Functions of secondary storage Secondary storage also called auxiliary storage or backing storage is used to store data and instructions when they are not being processed. Secondary storage is much cheaper than primary storage and is also used to back up for later use. Storage media Storage media are the physical hardware on which a computer keeps data, instructions and information. E. g. a CD, a DVD, a floppy disk, hard disk. Storage device Storage devices record and retrieve data, instructions and information to and from storage media. Accessing Files Serial access means the records are accessed in the order in which they were stored. To read a serial file a computer has to read each record until it reaches the one required. Sequential access means reading or writing data consecutively. With sequential access the records are in a particular order and the records are accessed in sequence. Magnetic tape is accessed sequentially. Direct access or random access it is possible to go directly to a record without having to look at any other records. Magnetic disks, optical disks Zip disks The Zip disk is a medium-capacity removable disk storage system that was introduced by Iomega in late 1994. Originally, Zip disks launched with capacities of 100  MB, but later versions increased this to first 250  MB and then 750  MB. It is a small portable disk. Zip drives are used to backup important data. Advantages * Inexpensive (general) * Can transfer large files from one computer to another * Backup Disadvantages * Cost more than DVd’s or CDR’s * Not many computers have zip drives any more Magnetic Tape Magnetic tape is a ribbon of plastic material coated with a metal oxide film on which data is recorded as magnetized or unmagnified spots. The magnetized spots represent ones and the unmagnetized spots represent zeros. In microcomputers tapes are stored in cartridges. On mainframe computers the tape is stored on reels or cartridges. Magnetic tape cartridges can hold up to 1. 6TB Advantages * Reusable * Easy to transport and store * Cheap * Used for backups or archives Disadvantages * Sequential access * Limited shelf life- 2 years * Must be stored properly in a suitable environment away from smoke dust and humidity * Difficult to update files(cannot make a change to a record without saving over the entire tape) The diagram for representing data on magnetic tape is in the textbook page 10 chapter 3. Floppy disks A 3. 5 inch floppy disk is a removal flexible plastic disk coated with a magnetized material. The disk is contained in a hard plastic case. The term floppy refers back to the 5. 24 â€Å"diskettes that were used in the 1980’s and were actually soft and flexible. Each disk contains a number of concentric rings known as tracks. The tracks are divided into pizza slices called sectors. Data is stored as magnetized or unmagnetized spots. They are used to transfer data between computers and for backup of files. They are slow to access because of slow rotation speeds. They hold 1. 44 MB hold data. Advantages * Direct access * Small * Portable * Suitable for backup * Erasable, reusable and rewritable Disadvantages * Small storage capacity * Limited shelf life * Slow read/write speeds Before a disk is used to stored data it must be formatted. Formatting is the process of preparing a disk to receive and store data. When a disk is formatted the following happens: * All data on the disk is from a erased * The surface of the disk is checked for any physical and magnetic defects. Care of floppy disks * Keep your disks away from liquids, dust, dirt Do not bend disks or handle them roughly * Take care when removing or inserting a disk from a computer’s disk drive * Keep disks in a disk box * Do not leave in the drive Fixed Hard disk Thin rigid, inflexible disks made of polished metal. The surface of each disk platter is covered with a substance that can be magnetized allowing data to be stored as magnetized or unmagnetized spots. Like a floppy a har d disk must be formatted to set up the tracks and sectors. It is faster than a floppy disk because it rotates at a much faster speed. Fixed hard disks can store up to 1 TB. Hard disk The access time from a spinning disk with one read/write head is a combination of: Seek time How long it takes the head to get to the right track Rotational delay or latency time How long it takes for the data to rotate under the head Transmission time The time taken to read the data and transmit it to the CPU. Advantages * Direct access * Fast data transfer speeds * Vast storage capacity * Up to one terabyte Disadvantages * Not Portable Another type of hard disk is the external hard disk. An external hard disk can be used if there is no space in the system unit to house another hard isk. It may come with its own power source and is connected to the system via a special cable. External hard disks can be easily removed and connected to another computer, thereby giving them a certain amount of portability and flexibility of use. Diagrams showing how data is stored on magnetic disks The data is stored in concentric rings called tracks. Disk Sector: each side of the disk is divided into wedge – shaped slices called sectors. A track sector is an area of intersection of a track and a sector. A cluster: is a set of track sectors. A hard disk is a set of hard metal disks mounted on the same spindle. A cylinder is a set of matched tracks on all the disks. Optical disks Disks read by laser lights. They are made up of a type of plastic polycarbonate. The data is stored on a layer inside the plastic. A metal coating usually aluminum reflects the laser light back to a sensor. Main types of optical disks are Cd’s, DVD’s, and blu -ray disks. Advantages * Direct access * Fast data transfer speeds * Vast storage capacity compared to diskettes * Portable * Unaffected by magnetic fields Disadvantages Data on a Cd-ROM cannot be changed * Access times are slower than hard drives CD- ROM Compact Disk read only Memory The user can only read or access what is on the disk, nothing can be changed. The data is encoded and read optically with a low intensity laser light. The data is represented as a series of pits and lands. A pit is a little depression formed by the laser burning into the data layer when the Cd is created. The land is the part between the pits. Reading a Cd is done by shining a laser light at the disk and detecting the changing reflective patterns. When it reflects off the smooth surface land it is interpreted as a 1 bit. When the laser enters a pit there is no reflection it is interpreted as a 0. The speed of a CD- ROM is indicated by a number followed by an X. e. g. 48X, 50X. The higher the number the faster the disk spins. CD’s are used to store software, multimedia. Stores approximately 750 MB. CD-R Compact disk recordable Allows the user to write data onto the disk ONCE using a burner. The disk then becomes a CD-ROM and the contents cannot be changed. Used to create music CD’s. Stores approximately 750 800 MB CD-RW Compact disk Rewritable The data layer uses a phase changing metal alloy film. By using a high intensity laser light the film can be melted to level out the marks made by the laser burner when the data was stored. New data can be recorded using a low intensity light to burn new data. The user can erase and write to the disks many times. Used for backup. DVD Digital versatile Disk DVD- ROM DVD – R DVD – RW They are similar to CD’s but can hold more data. The tracks on a DVD are closer together thus allowing more tracks. Pits are smaller on a DVD. This means many more pits. The DVD stores approximately 4. 7 GB 17GB. Used to store Movies. CD –ROM –ROM and DVD – ROM ( more specific advantages and disadvantages Advantages * They hold far more data than floppy disks. * They are less expensive than hard disks Disadvantages * They data transfer rate and data access times are slower than for hard disks. CD-R and DVD –R ( more specific advantages and disadvantages) Advantages * CD-R and DVD-R are cheaper than the RW’s Disadvantages * They can only. be recorded once, so if an error occurs then the disk has to be thrown away CD-RW and DVD –RW (specific advantages and disadvantages) Advantages * They can be used many times Disadvantages * It is possible to overwrite data Blu- Ray Capacity 25 GB 50GB. It is suitable for recording, rewriting high definition videos. While Cd’s and DVD’s rely on a red laser to read and write, Blu- ray uses a blue violet laser. Advantages * Direct access * Fast data access speed is faster than with other optical media * Vast storage capacity * Portable * Users can copy the content of a disk a limited number of times Disadvantages * Expensive Care of Optical disks * CD’s DVD’s should be stored in their cases when not in use to prevent them from being scratched or getting dirty. * Avoid soiling the surface of the disk – hold it by the edge * Keep your CD’s clean by gently wiping both sides with a clean damp cloth from the centre to the outer edge * Do not write on the top side of the Cd with a ball point pen or other hard object as this can damage the CD. Use a CD marker instead. * Do not expose a Cd to high temperature or humidity for an extended period of time. Flash Memory It is based on EEPROM (electrically erasable programmable read only memory) Flash memory is Non- volatile. Reading and writing to flash memory is very fast. This is because data in flash memory can be erased a block at a time instead of only a single byte at a time as is the case with EEPROM. Flash memory got its name because a block of memory cells is erased in a single action of flash. Flash memory cards are used in digital cameras. These include compact flash, SmartMedia, MultiMediaCard. Flash memory is also used in PC’s, cell phones PDA’s, MP3 players and video game consoles. Capacity -2GB – 16 GB. Advantages * Small * Portable * High data transfer speeds to PC’s * Large storage capacity * Direct access Disadvantages * Limited storage compared to a hard disk * May need special software to be used with PC’s USB drive This is a type of flash memory storage device which is called flash drive, flash pen, thumb drive, key drive, USB drive. They are small and plug into a USB port. Capacity 16 GB. Advantages * Easy to use * Convenient * Large storage capacity * Plug and play * Fast access * Direct Access Disadvantages * Easy to lose * Not possible to write protect the data