COMP 314 - U3C12

Operating System Concepts, Seventh Edition

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COMP 314 – Chapter 12 (Mass Storage Structure) Overview of Mass-Storage Structure (451) Magnetic Disks These provide the bulk of secondary storage for modern computer systems. Conceptually disks are simple: Each disk platter has a flat circular shape, like a CD. Plat diameters range from 1.8 to 5.25 inches. The two surfaces of the platter are covered in magnetic material. A read-write head “flies” just above each surface of each platter. (452) The heads are attached to a disk arm that moves the heads as a unit. The surface of a platter is logically divided into circular tracks, which are subdivided into sectors. The set of tracks that are at one arm position makes up a cylinder. There may be thousands of concentric cylinders in a disk drive, and each track may contain hundreds of sectors. When a disk is in use, a drive motors spins it at high speeds (usually 60 to 200 times per second). Disk speed has two parts: The transfer rate is the rate at which data flow between the drive and the computer. The positioning time, sometimes called random-access time , consists of the time to move the disk arm to the desired cylinder, called the seek time , and the time for the desired sector to rate to the disk head, called the rotational latency . Because the disks head flies on an extremely thin cushion of air, there is a danger of it colliding with the surface (called a head crash). If damage is done to the surface, this cannot be repaired, and can require the disk to be replaced. A disk drive is attached to a computer by a set of wires called an I/O bus . (453) Several kinds of buses are available: enhanced integrated drive electronics (EIDE), advanced technology attachment (ATA), serial ATA (SATA), universal serial bus (USB), fiber channel (FC), and SCSI buses. The data transfers on a bus are carried out by special electronic processors called controllers. The host controller is the controller at the computer end of the bus. A disk controller is built into each disk drive. To perform an I/O operation, the computer places a command into the host controller, typically memory mapped I/O ports. The host controller sends a command to the disk controller, which operates the disk drive. Disk controllers usually have a built-in cache. Magnetic Tapes (453) Has large storage, but is relatively slow compared to main memory and magnetic disks. They are about 1000 times slower, and thus only good for backups. Disk Structure (454) Modern disk drives are addressed as large one-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The size of a logical block is usually 512 bytes, but some can be low-level formatted to 1024.
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This note was uploaded on 07/18/2011 for the course COMP 314 taught by Professor Dr.dunwei(grant)wen during the Spring '11 term at Open Uni..

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COMP 314 - U3C12 - COMP 314 Chapter 12(Mass Storage...

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