A Disk Drive with Multiple Tracks: Seek Time

In the figure, the head is currently positioned over the innermost track, which contains sectors 24 through 35. The next track over contains the next set of sectors (12 through 23), and the outermost track contains the first sectors (0 through 11).

To understand how the drive might access a given sector, we now trace what would happen on a request to a distant sector, e.g., a read to sector 11. To service this read, the drive has to first move the disk arm to the correct track (in this case, the outermost one), in a process known as a seek. Seeks, along with rotations, are one of the most costly disk operations.

The seek, it should be noted, has many phases:

• First, an acceleration phase as the disk arm gets moving.
• Then coasting as the arm is moving at full speed.
• Then deceleration as the arm slows down
• Finally settling as the head is carefully positioned over the correct track.

The settling time is often quite significant, e.g., 0.5 to 2 ms, as the drive must be certain to find the right track (imagine if it just got close instead!).

After the seek, the disk arm has positioned the head over the right track. A depiction of the seek is found in the figure below.

As we can see, during the seek, the arm has been moved to the desired track, and the platter of course has rotated, in this case about 3 sectors. Thus, sector 9 is just about to pass under the disk head, and we must only endure a short rotational delay to complete the transfer.

When sector 11 passes under the disk head, the final phase of I/O will take place, known as the transfer, where data is either read from or written to the surface. And thus, we have a complete picture of I/O time: first, a seek, then waiting for the rotational delay, and finally the transfer.