Disk Storage

         The data (record) is stored in a database system. The database system uses several types of memory to stored data in a database system. Three types of memory are most common. Disk storage, magnetic tape, main memory are a major type of memory but optical disk and other devices are also play a vital role in database system. The brief description of disks storage is here to explain the characteristics of disk storage.

                 Disk storage is nonvolatile it means that the data is not erased when system is shut off. The portion of a data can be accessed in any order so disk is a direct access storage device (DASD).  The disk is available in 3 ½, 5 ¼, 8 or 14 inches. The composition of magnetic disks is a circular metal (plastic platters) coated with a film of magnetized material. Disk drives are used to stored and read a data from a disk. 

How disk work to store data?? Data is stored by tracks on the surface of the disk. In movable-head disk unit read/write head is attached to an access arm (access arm is a rod that can be moved on the surface). Disk is rotated when read/write head is position over a track then the entire track is passes under the read/write head. The data can be stored and read at any position because of the arm movement and disk rotation. In fixed-head units for every track there is a read/write head.

Dust and smoke particle cause a major problem when read/write head were moved closer to the tracks. But now Sealed the disk and read/write head in a dust proof housing the Winchester technology reduces dust and smoke contamination problem. What is a Winchester technology??? Who Winchester technology reduces dust and smoke problem?? Winchester cartridges consist of high-capacity rigid disk in sealed housing and Winchester drives are high capacity unit that hold portable or permanently mounted sealed disks. 

The set of tracks that are in the corresponding positions on their surfaces is called a cylinder. The number of cylinders in a disk pack is obviously the same as the number of tracks on each surface.

The time to transfer data from disk to main memory is determine by the following

Seek time:  the time that read/write head takes to reach the correct track. The seek time is shorter when the desire position is near to the head current position.

Rotational delay time/latency time: the time it takes to rotate the disk

Head switching time: the time to select the correct read/write head for the surface needed.

Transfer time: the time to move data from the disk to main memory.

The factors that are important for determining the capacity of the disk are

Composition of the disk, it’s size, data density, number of the bits stored per inch of the track, and number of track per inch surface.

Hard disks rotate faster than floppy disks, hard disks rotate in the range up to about 4700 revolutions per minute and floppy disks rotate in the range of 300 to 600 revolutions per minute.

The mechanical movement in using disks cause a crushed of head, the disk unit fail and destroy a data cause a major problem. Fixed-head disk unit may be used when time is crucial because the spinning of disks and the movement of access arms caused delays of data access.