Magnetic Disks - Magnetic Disks Physical Characteristics of...

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Magnetic Disks Physical Characteristics of Disks Performance Measures of Disks Optimization of Disk-Block Access Physical Characteristics of Disks 1. The storage capacity of a single disk ranges from 10MB to 10GB. A typical commercial database may require hundreds of disks. 2. Figure 10.2 shows a moving-head disk mechanism. o Each disk platter has a flat circular shape. Its two surfaces are covered with a magnetic material and information is recorded on the surfaces. The platter of hard disks are made from rigid metal or glass, while floppy disks are made from flexible material. o The disk surface is logically divided into tracks , which are subdivided into sectors . A sector (varying from 32 bytes to 4096 bytes, usually 512 bytes) is the smallest unit of information that can be read from or written to disk. There are 4-32 sectors per track and 20-1500 tracks per disk surface. o The arm can be positioned over any one of the tracks. o The platter is spun at high speed. o To read information, the arm is positioned over the correct track. o When the data to be accessed passes under the head, the read or write operation is performed. 3. A disk typically contains multiple platters (see Figure 10.2). The read-write heads of all the tracks are mounted on a single assembly called a disk arm , and move together. o Multiple disk arms are moved as a unit by the actuator . o Each arm has two heads, to read disks above and below it. o
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Magnetic Disks - Magnetic Disks Physical Characteristics of...

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