MAGNETIC DISKS

MAGNETIC DISKS

Magnetic Disk are used for storing large amounts of data. The storage capacity of a single disk ranges from 10 MB to 20 GB. Magnetic disks support direct access to a desired location and are widely used for the database applications. A DBMS provides access to data on disk ; applications of DBMS should not worry about, whether data is in main memory are disk.

    The most basic unit of data on the disk is a single bit of information. By magnetizing an area on disk in certain ways, one can make a bit value to represent either in 1 or 0. To code information, bits are grouped into bytes (bytes or characters). Byte sizes are equal to 4 to 8 bytes, depending on the computer and the device. We assume that one character is stored in a single byte, and we used the terms byte and character interchangeably. The capacity of a disk is the number of bytes it can store, which is very large. small floppy disk used with micro computers holds from 400 KB to 1.5 MB; hard disks for micro - typically hold from several hundreds MB to few GB: and large disk packs used with minicomputers and mainframes have capacities that range up to a few tens or hundred of  GB. Disk capacities continue to grow as technology improves. Data is stored on disk in units called disk blocks. A disk block is a contiguous sequence of bytes and is the unit in which data is written to a disk and read from a disk.

         Blocks are arranged in concentric rings called tracks, on one or more platters. Tracks can be recorded on one or both surfaces of platter.

We refer to platters as single - sided or double - sided accordingly. 

The set of all tracks with the same diameter is called a cylinder, because the space occupied by these tracks is shaped like a cylinder; a cylinder contains one track per platter surface.