Data Track- Concentric circular (not perfectly) tracks on the disk where binary bits are stored sequentially.
• Track Pitch - Distance between two adjacent tracks. • Track Density - Inverse of track pitch, i.e., the number of tracks in unit length of radius of the disk. It is usually defined in units of Tracks per 4 CHAPTER 1. INTRODUCTION Inch* (TPI).
• Bit Density - Number of bits recorded per unit length of a track, defined in units of Bits per Inch (BPI).
• Areal Density - Number of bits recorded per unit area of the disk surface. It is equivalent to the product of track density and bit density, and is defined in units of bits per square inch. • Access Time - This is the time required to retrieve a block of data from the disk and is equal to sum of seek time and average latency, both defined below.
• Seek Time - Time taken by the head positioning servomechanism to move the read/write head from one track to another. • Single Track Seek Time - Seek Time for moving the head from one track to the adjacent track. • Average Seek Time - Exact Seek Time depends on the seek length, i.e., the difference between the initial track and destination track. Average Seek Time is an average of seek times for all possible seek lengths. • One-third Stroke Seek Time - Seek Time for moving head over a distance equal to one third of the maximum stroke. • Latency - The process of reading or writing can not be initiated immediately after positioning the head over the destination track as the exact location of the track may not be under the head at that moment. The read-write process must wait for some time before the desired sector of data is available. This waiting time is the latency, and it contributes to the access time.
• Average Latency - Each data retrieval process has a different latency. Average latency is the time equal to half the time required for one revolution of the disk.