Monday, 28 July 2014

COBOL How Data is stored on a disk drive.

We will discuss the concepts and terms that apply to the way files are stored on disk drives and the way data is accessed on disk drives. 

A Disk Drive contains a stack of metal platters (or disks) that are coated with a metal oxide. Data is recorded on one or both sides of each of the disks.

Data is stored on each recording surface of a disk in concentric circles called tracks. Each track is divided into sectors, and each sector has a unique disk address. When the disk drive reads or writes data, it reads or writes one or more complete sectors.

When the disk drive is in operation, the disks rotate around the spindle at high speeds. A common speed, for example, is 3600 rotations per minute.

As you can see, each disk drive consists of disks that are stacked on a spindle within the drive; each magnetic recording surface on a disk is divided into tracks; and each track is divided into sectors.

This is conceptually true whether the disk drive is for a PC, a mid-range system, or a mainframe. 

What differs from one disk drive to another is the number of disks, the number of tracks on each recording surface, the number of sectors in each track, and the number of bytes in each sector. 

Example :  A drive has 10 recording surfaces with 10,000 tracks per surface, 25 sectors per track, and 4000 bytes in each sector, the storage capacity of the disk is one billion bytes of data (a gigabyte). Today, disk drives with that capacity or more are common on PCs, and the disk drives for mainframes have capacities that are many times larger.

A disk drive on an IBM mainframe is called a DASD, which is short for direct access storage device. You should also know that the term sector isn’t used for the addressable storage units of DASDs. The concept, however, is the same. The tracks on DASDs are divided into fixed-length units with unique disk addresses. 

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