Figure 4.1 shows the major components of a disk.
There can be any number of platters. The platters rotate together around the spindle. Spindle speed range from 3,600 revolutions per minute (rpm) to more than 7,200 rpm. The faster the speed, the higher the transfer rate and the less time it takes for data to rotate under the head (this time is known as the rotation latency). Each platter has a read/write head, but there's only one data channel for the whole drive. The channel can be switched to whatever head is doing the read or write. [RW94] The read/write head can only read or write to a sector when that sector is immediately under the head. [Jen96]
Data is stored in concentric circles known as tracks on a disk. A stack of tracks at a common distance from the spindle is called a cylinder. In their paper on drive modeling, Ruemmler and Wilkes suggest that track alignment tolerances are so fine that considering cylinders in a simulation is irrelevant and the tracks on each platter should be considered independently. [RW94]
A seek, or head movement, consists of four actions: [RW94]
For short seeks of up to four cylinders, the settle time of one to three milliseconds will dominate. A full seek may not even occur. For seeks of up to 400 cylinders, the speedup time will dominate and that time is proportional to the square root of the seek distance plus the settle time. In seeks of greater than 400 cylinders, the coast time dominates and that time is proportional to distance plus a constant overhead. The fraction of time due to settle time increases as disks become smaller and track density increases. [RW94]
Settle times for reads and writes may differ as much as 0.75 ms because modern drives will attempt disk reads before the settle operation has completed. [RW94]
A track-following system makes the fine adjustments to the head position at the end of the seek and keeps the head on the desired track. It does this by using information recorded on the disk at manufacturing time. The information for any specific platter is either recorded on that platter or on a dedicated platter for the whole disk. Disks with few platters generally contain the positioning information on each platter. The track-following system is also responsible for head switches that occur when the controller switches the data channel from one platter to another or when the arm is moved from the last track of a cylinder to the first track of the next. The first is about one third to one half of the time to perform a settle operation (0.5-1.5 ms), while the second requires the same amount of time as a settle operation. [RW94]
As can be seen from the above discussion, to determine the time required to access any sector on a disk, it is necessary to consider both seek time and rotational latency. If the head is over the desired track and sector, then the access time is approximately zero. If the head is over the correct track, but not the correct sector, then the access time is approximately the rotational latency. Finally, if the head is not over the correct track, the average access time will be the total of rotational latency plus seek time. The average access time is presently eight to 25 milliseconds. [Jen96]