Mass-Storage Structure In Operating System

In this post, we will discuss about a general overview of the physical structure of secondary and tertiary storage devices.

 Magnetic Disks :-

    Magnetic disks provide the bulk of secondary storage for modern computer systems. Each disk platter has a flat circular shape  like a CD. Generally platter diameters range from 1.8 to 3.5 inches. The two surfaces of a platter are covered with a magnetic material.We store information by recording it magnetically on the platters.

A read–write head “flies” just above each surface of every platter. The heads are attached to a disk arm that moves all 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 the disk is in use, a drive motor spins it at high speed. Most drives rotate 60 to 250 times per second, specified in terms of rotations per minute (RPM).

    Disk speed has two parts. The transfer rate is the rate at which data flow between the drive and the computer.

 

The positioning time, or random-access time, consists of two parts: the time necessary to move the disk arm to the desired cylinder, called the seek time, and the time necessary for the desired sector to rotate to the disk head, called the rotational latency. 

Because the disk head flies on an extremely thin cushion of air, the head will sometimes damage the magnetic surface. This accident is called a head crash. A head crash normally cannot be repaired; the entire disk must be replaced.

Removable magnetic disks generally stored in a plastic case to prevent from damage while not in the disk drive. Other forms of removable disks include CDs, DVDs, and Blu-ray discs etc. A disk drive is attached to a computer by a set of wires called an I/O bus. Several kinds of buses are available, including advanced technology attachment (ATA), serial ATA (SATA), eSATA, universal serial bus (USB), and fibre channel (FC).

 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 a disk I/O operation, the computer places a command into the host controller by using memory-mapped I/O ports.The host controller then sends the command via messages to the disk

controller, and the disk controller operates the disk-drive hardware to carry

out the command. 

 Magnetic Tapes :-

Magnetic tape was used as an early secondary-storage medium. Although it is relatively permanent and can hold large quantities of data, its access time is slow compared with that of main memory and magnetic disk. In addition, random access to magnetic tape is about a thousand times slower than random access to magnetic disk, so tapes are not very useful for secondary storage.

Tapes are used mainly for backup, for storage of infrequently used information, and as a medium for transferring information from one system to another. A tape is kept in a spool and is wound or rewound past a read–write head. Moving to the correct spot on a tape can take minutes, but once positioned, tape drives can write data at speeds comparable to disk drives. Tapes and their drivers are usually categorized by width, including 4, 8, and 19 millimeters and 1/4 and 1/2 inch.

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