Hardware for Storing and Retrieving
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Random or Sequential Access
The 'random' accessing of data involves directly retrieving data from a source without analysing any data before it. In other words, the location of the data is know so it can be immediately found instead of being searched for by analysing each set of data until a match is found. This method is very fast and efficient. This method is typically used by devices such as hard drives and flash memory
The sequential method of accessing data involves searching through data logically until the required data is found. Each item before the required data is analysed until the correct data is found. Using this method is very slow and inefficient. This is typically used by magnetic tapes.
The 'random' accessing of data involves directly retrieving data from a source without analysing any data before it. In other words, the location of the data is know so it can be immediately found instead of being searched for by analysing each set of data until a match is found. This method is very fast and efficient. This method is typically used by devices such as hard drives and flash memory
The sequential method of accessing data involves searching through data logically until the required data is found. Each item before the required data is analysed until the correct data is found. Using this method is very slow and inefficient. This is typically used by magnetic tapes.
Volatile or Non-Volatile
Volatile storage methods require a constant current of electrical power to retain data or else the data will be permanently lost. This is typical of dynamic RAM which is installed in almost every computer; soon after powering off your system the memory loses any data it contains.
Non-volatile storage methods, unlike volatile methods, do not need a constant power source to retain data. They instead store data in ways that hardly deteriorate over time. An example of this is the magnetic storage method used in hard drives. This storage method can also be considered a ‘permanent’ storage method as any data stored takes decades to deteriorate.
Volatile storage methods require a constant current of electrical power to retain data or else the data will be permanently lost. This is typical of dynamic RAM which is installed in almost every computer; soon after powering off your system the memory loses any data it contains.
Non-volatile storage methods, unlike volatile methods, do not need a constant power source to retain data. They instead store data in ways that hardly deteriorate over time. An example of this is the magnetic storage method used in hard drives. This storage method can also be considered a ‘permanent’ storage method as any data stored takes decades to deteriorate.
Operation of Secondary Storage Hardware
Magnetic Storage
Data is stored magnetically by changing the polarity of the medium used to store data. By using tiny electromagnets, data can we written or read from magnetic media by using the principle of polarity reversal. A change of polarity in the medium represents a binary 1 and no change a 0. This occurs because at the point of polarity change, the magnetic field is strongest which can be made or detected by the tiny electromagnets involved in handling magnetic data.
Data is stored magnetically by changing the polarity of the medium used to store data. By using tiny electromagnets, data can we written or read from magnetic media by using the principle of polarity reversal. A change of polarity in the medium represents a binary 1 and no change a 0. This occurs because at the point of polarity change, the magnetic field is strongest which can be made or detected by the tiny electromagnets involved in handling magnetic data.
Storing or Writing Magnetic Data
The electromagnet involved in the storage of magnetic data is fairly simple. It is comprised of an incomplete loop of soft magnetic material with a coil of copper wire coiled around it. When current flows through the copper wire it magnetises the magnetic material in one of two ways dependant on the direction of current flow through the copper coil. The magnetic material at the two points where the loop is incomplete becomes either north or south which can be reversed by reversing the current flow. This is also the place where the magnetic media passes under, being magnetised in different ways as it moves storing data on the disk. |
Retrieving or Reading Magnetic Data
Read heads are made up of MR materials which are particularly sensitive to magnetic fields. When a constant current in fed into the material, the resistance of the material changes based on the strength of the magnetic field it is passing over; A stronger magnetic field causes less resistance and hence allows more current to pass through the material representing a 1, the opposite occurs for a 0. This property allows the read head to detect minor changes in a magnetic field and interpret it as binary data.
Read heads are made up of MR materials which are particularly sensitive to magnetic fields. When a constant current in fed into the material, the resistance of the material changes based on the strength of the magnetic field it is passing over; A stronger magnetic field causes less resistance and hence allows more current to pass through the material representing a 1, the opposite occurs for a 0. This property allows the read head to detect minor changes in a magnetic field and interpret it as binary data.
Magnetic Hard Disks
Magnetic hard disks are made of multiple precision aluminium or glass platters with a hard layer of magnetic material (mainly iron oxide) on top for storing magnetic data. This is then covered by a layer of carbon and then lubricant to increase the durability of the disk and reduce corrosion of the magnetic layer. Each disk is double sided, requiring two read/write heads per disk. A typical hard disk will contain multiple disks to maximise storage space. These disks are then sealed within a casing to protect all the parts inside from dust and humidity. Each disk is arranged into a series of tracks and sectors that divide the disk into equal portions of storage space. Each track is a concentric circle arranged so there a multiple tracks from the centre to the edge of each disk. Every track is then split into multiple parts so each part contains the same amount of storage space. |
Data is written or read off a magnetic hard disk by tiny read/write heads mounted on the end of a head arm that is moved over the correct portion of the disk by the actuator assembly. The actuator assembly controls the position of all the arms inside the disk moving them as a single arm; this means that only one arm in the whole hard drive will be in the correct place and operational at any time. While this occurs, all the disks are spun by the spindle motor at speeds of up to 15000 RPM, allowing data to pass under the read/write heads.
Magnetic Tape
Magnetic tape is a sequential access storage medium mainly used today for backup purposes. This is because they are the most cost effective way for storing enormous amounts of data. Despite this, they are slow and data can only be accessed in a sequential fashion severely increasing the time it takes to locate data. These are two types of magnetic tape systems, helical and linear.
Magnetic tape is a sequential access storage medium mainly used today for backup purposes. This is because they are the most cost effective way for storing enormous amounts of data. Despite this, they are slow and data can only be accessed in a sequential fashion severely increasing the time it takes to locate data. These are two types of magnetic tape systems, helical and linear.
Optical Storage
Data on an optical storage medium is stored as reflections, an example of this is a CD-ROM. Data on a CD-ROM is represented as binary based on whether the surface of the disk has changed from a poor to a good reflective surface and visa-versa.
Data on an optical storage medium is stored as reflections, an example of this is a CD-ROM. Data on a CD-ROM is represented as binary based on whether the surface of the disk has changed from a poor to a good reflective surface and visa-versa.
When a disk is read a combination of things occur. First the spindle assembly rotates the disk at the appropriate speed for the LASER to read the data before the read head assembly is moved into position. When in position, a laser beam is shot at the part of the disk to be read hitting either a pit (bad reflective surface) or a landing (good reflective surface). Depending on what the laser hits will determine the amount of light reflected off of the disk. The reflected light from the disk is then reflected again by a Beam splitter prism onto an opto-electrical cell. This cell picks up the amount of light hitting it and converts this into binary data where a change in reflectivity is read as a 1 and no change is read as a zero.
When writing data to an optical disk one of two processes occur depending on the type of disk that is being written on, a write once or rewritable disk. Write once technologies use a clear dye that is sandwiched between a layer of reflective metal and clear plastic as the method of storing data. During the process of ‘burning’ data, the laser heats up the dye enough so that the clear dye becomes opaque which also blocks any light from contacting the reflective surface behind it creating a pit. |
Rewritable media instead use a reflective crystalline compound sandwiched between two insulating layers for data storage. When the crystalline is heated to a temperature of 500-700°c its crystalline structure breaks down and loses its reflective properties creating a pit. Despite this, the reflective properties can be restored if the crystalline is reheated to 200°c after cooling creating a landing.
Flash Memory
Flash Memory is a non-volatile storage method that utilises the ability to trap electrons in a circuit to store data; where an electron represents a 1 and no electron represents a 0. Flash memory is also a solid state memory which means that it contains no moving parts making it a reliable and silent high-speed storage device. It is also versatile in its applications as it is small and can withstand greater temperature extremes than most other storage mediums. However, this kind of memory is significantly less cost effective than any other storage device due to its high price tag and small storage space. |
Network Storage
RAID
The RAID method of storage utilises an array of multiple hard drives to achieve speed and reliability. Speed is achieved through a method called stripping. This is where data is stored evenly through all the hard disks attached so that data is retrieved from multiple hard drives at once, considerably improving speed compared to retrieving data from a single drive. In addition to this, data stored in this way is protected. Any data stored is mirrored onto multiple hard drives so that if one hard drive was to fail no data loss would occur as a copy of the data was stored on at least one other hard drive. Essentially, RAID method of network storage is a fast, expensive, large scale back-up system.
Tape Libraries
Tape libraries are an automated system designed for making large scale backups. These systems contain multiple tape drives and an even larger number of tapes. Tapes are automatically inserted into each tape drive when necessary allowing the system to perform a constant backup of data.
RAID
The RAID method of storage utilises an array of multiple hard drives to achieve speed and reliability. Speed is achieved through a method called stripping. This is where data is stored evenly through all the hard disks attached so that data is retrieved from multiple hard drives at once, considerably improving speed compared to retrieving data from a single drive. In addition to this, data stored in this way is protected. Any data stored is mirrored onto multiple hard drives so that if one hard drive was to fail no data loss would occur as a copy of the data was stored on at least one other hard drive. Essentially, RAID method of network storage is a fast, expensive, large scale back-up system.
Tape Libraries
Tape libraries are an automated system designed for making large scale backups. These systems contain multiple tape drives and an even larger number of tapes. Tapes are automatically inserted into each tape drive when necessary allowing the system to perform a constant backup of data.