A Simple Disk Drive

This track has just 12 sectors, each of which is 512 bytes in size (our typical sector size, recall) and addressed therefore by the numbers 0 through 11. The single platter we have here rotates around the spindle, to which a motor is attached.

Of course, the track by itself isn’t too interesting; we want to be able to read or write those sectors, and thus we need a disk head, attached to a disk arm, as we now see in the figure below.

In the figure, the disk head, attached to the end of the arm, is positioned over sector 6, and the surface is rotating counter-clockwise.

Single-track latency: the rotational delay

To understand how a request would be processed on our simple, one-track disk, imagine we now receive a request to read block 0. How should the disk service this request?

In our simple disk, the disk doesn’t have to do much. In particular, it must just wait for the desired sector to rotate under the disk head. This wait happens often enough in modern drives, and is an important enough component of I/O service time, that it has a special name: rotational delay (sometimes rotation delay, though that sounds weird). In the example, if the full rotational delay is $R$, the disk has to incur a rotational delay of about $\frac{R}{2}$ to wait for $0$ to come under the read/write head (if we start at 6). A worst-case request on this single track would be to sector 5, causing nearly a full rotational delay in order to service such a request.