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The Basics: TPI and BPIA disk drive is, at its simplest, a delivery mechanism for persistent storage using magnetic recording techniques. The objectives of the drive design process are to improve the capacity, reliability, and performance of this mechanism at a minimal cost. A drive can be thought of as a three-dimensional space of recorded information. The surface of a disk provides two dimensions, and the stack of disks that make up the drive is the third dimension. Capacity can be increased by adding disks. This drives up cost and causes more difficulties with increasing areal density as disk crowding increases. Increased vibration in the spindle, susceptibility to external vibration, and internal disturbance resulting from turbulent airflow will all increase with more disks in a given space. For decades disk-drive capacity has been increased by reducing the spacing between data tracks, or track pitch, measured in tracks per inch (TPI), and increasing the linear density of the bits along a track, measured in bits per inch (BPI), as shown in Figure 1. Figure 1
The product of these terms is areal density, measured today in gigabits per square inch. A state-of-the-art disk drive might have 30-plus gigabits per square inch. The challenge of designing head, media, and signal-processing systems to achieve higher areal density dominates the development of any disk drive. Figure 2 shows the history of areal density growth. Figure 2
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