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According to Mustakallio, LK 2010, a magnetic disk is a disk that uses magnetic fields to store data. The most common types of magnetic disks are hard disks and floppy disks. Although there are other types of disks used for data storage, such as CD-ROMs, magnetic disks are the type most frequently referred to simply as "disks.
A magnetic disk device is configured to access a disk by a head according to a command from a computer. The magnetic disk device comprises a communication module configured to receive a command from the computer; an information generator configured to generate first information indicating access situation of the disk based on the command received by the communication module; and a controller configured to control a driving speed of at least one of the head and the disk based on the first information generated by the information generator.
Similarly, storage today more commonly refers to mass storage - optical discs forms of magnetic storage like hard disk drives, and other types slower than RAM, but of a more permanent nature. Historically, memory and storage were respectively called main memory and secondary storage. The terms internal memory and external memory are also used distinguish between memory and storage devices. Vitter, JS 2008 maintained that the most important part of every computer is the central processing unit (CPU, or simply a processor), because it actually operates on data, performs any calculations, and controls all the other components. In practice therefore, almost all computers use a variety of memory types, organized in a storage hierarchy around the CPU, as a trade-off between performance and cost. Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. This traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit, Mike, C 2004.
Reason for Slow Performance in Magnetic Disk
There are several factors that could slow down the performance of a magnetic disk. Chief among them, as claimed by PCTECH Guide, is the rotational speed of the platters. Disk RPM is a critical component of hard drive performance because it directly impacts the latency and the disk transfer rate. The faster the disk spins, the more data passes under the magnetic heads that read the data; the slower the RPM, the higher the mechanical latencies. Hard drives only spin at one constant speed, and for some time most fast EIDE hard disks span at 5,400rpm, while a fast SCSI drive was capable of 7,200rpm. In 1997 Seagate pushed spin speed to a staggering 10,033rpm with the launch of its UltraSCSI Cheetah drive and, in mid 1998, was also the first manufacturer to release an EIDE hard disk with a spin rate of 7,200rpm.
The introduction of Pegasus II SCSI drive by Hitachi in 1999 that spins at an amazing 12,000rpm - which translates into an average latency of 2.49ms, did little to help the situation. Hitachi has used an ingenious design to reduce the excessive heat produced by such a high spin rate. In a standard 3.5in hard disk, the physical disk platters have a 3in diameter. However, in the Pegasus II, the platter size has been reduced to 2.5in. The smaller platters cause less air friction and therefore reduce the amount of heat generated by the drive.
Ways of Improving Performance
Bertram, HN, and Cuddihy, EF, 1982, maintained that the under listed factors if adhered to can improve the performance of magnetic disks. They include:
Low Humidity is the most dangerous environmental factor. Water is the agent of the main chemical deterioration process of polymers: hydrolysis. Additionally, high humidity values (above 65% RH) encourage fungus growth, which literally eats up the pigment layer of magnetic tapes and floppy disks and also disturbs, if not prevents, proper reading of information. Therefore disks should be stored in places with low humidity of less than 40%.
Low Temperature is responsible for dimensional changes of carriers, which is a particular problem for high density tape formats. Temperature also determines the speed of chemical processes: the higher the temperature, the faster a chemical reaction (e.g. hydrolysis) takes place; the lower the temperature, the slower the chemical reaction. Low temperatures (10-20*C) must be maintained in areas where disk are used.
Stray magnetic fields are the natural enemy of magnetically recorded information. Sources of dangerous fields are dynamic microphones, loudspeakers and head sets. Also magnets used for magnetic notice boards etc, possess magnetic fields of dangerous magnitudes. By their nature, analogue audio recordings, including audio tracks on video tapes, are the most sensitive to magnetic stray fields. Analogue video and all digital recordings are less sensitive. For the safeguarding of analogue audio recordings is necessary to keep to the following maximum magnetic stray fields:
- AC fields: 5 Oe (Oersted) = 400 A/m (Ampere per metre)
- DC fields 25 Oe = 2000 A/m.
Magnetic disk storage is rapidly taking over the primary role of providing the storage, retrieval and the world-wide distribution of information and knowledge in our society via the Internet.
Every shortcoming in terms of performance, speed and capacity will always remain a challenge to manufacturers which they will eagerly accept.