It is more efficient for the processor to access and store information in semiconductor RAM. The average access time (time taken by the process of recording or retrieval of data) is the shortest for this type of memory. However they are the most expensive, constitute the least of the storage volume associated with an information processing system, and form the highest level of the storage pyramid shown in Figure 1.1. The lowest level of this pyramid consists of removable storage devices such as magnetic tapes, zip-disks and floppy disks using magnetic recording, and CD-ROM, DVD etc employing optical recording technology. Removability is the main advantage offered by this class of storage devices. Magnetic tapes and floppy disks are cheap but very slow and, therefore, not suitable for on-line direct access of data or programs. Optical disks are widely used for applications like program distribution, library and archive, entertainment systems etc, but they are not suitable for on-line storage due to their slow performance and high cost per read/write element. This segment of storage market was dominated by magnetic tapes in the early days of computing, but the emergence of optical recording technology caused the tapes to be replaced gradually by more cost-effective CD-ROM, DVD etc. Hard disk drives sit in the middle of the storage pyramid, between the semiconductor memories and removable drives, and occupy the non-removable on-line data storage niche. They provide direct access to large amounts of non-volatile storage (no power is required to preserve the data). Speed of data access in HDD is much higher than the removable, non-volatile storage, and its cost per gigabyte is only a fraction of that of non-volatile, direct access semiconductor memory such as “flash”. Hard disk drives are also known as Direct Access Storage Devices (DASD, pronounced as daz-dee).
In hard disk drives, a binary bit is stored in a tiny segment of the surface of a circular disk by magnetizing the medium coated on the surface with the help of an inductive head. In a majority of hard disk drives, the disk is spun at constant angular velocity by a spindle motor when the bits are written, and the head traces a circular path (Track ) on the spinning disk. Saturated magnetization of the media is used and it is magnetized in one of the two possible polarizations. The transitions between two opposite polarizations in the magnetic medium can be sensed by a sensor held over the track of a spinning disk. The disks are spun at the same speed during both writing and reading. The read head and write head are fabricated on a single slider whose surface facing the disk is profiled such that an air bearing surface (ABS) is produced between the spinning disk and the slider. As a result, the slider is lifted and is not in contact with the disk. The separation between the slider and the spinning disk, known as the flying height, is maintained as constant as possible. Characteristics of the flying height depend on many factors such as profile of the slider surface, smoothness of the disk surface, rotating speed of the disk etc. Flying height has direct effect on the achievable areal density - a key parameter defining the storage capacity and is equal to the number of bits recorded in unit area of the disk surface. Demand for higher areal density has always been and still is the driving force behind the dramatic growth of the magnetic storage technology. Areal density in magnetic recording has grown by a factor of 5, 000, 000 over last four decades.