5.4.2.3. Locality of Reads/Writes

Although not strictly constrained to a multi-requester environment, this aspect of hard drive performance does tend to show itself more in such an environment. The issue is whether the I/O requests being made of a hard drive are for data that is physically close to other data that is also being requested.

The reason why this is important becomes apparent if the electromechanical nature of the hard drive is kept in mind. The slowest component of any hard drive is the access arm. Therefore, if the data being accessed by the incoming I/O requests requires no movement of the access arm, the hard drive is able to service many more I/O requests than if the data being accessed was spread over the entire drive, requiring extensive access arm movement.

This can be illustrated by looking at hard drive performance specifications. These specifications often include adjacent cylinder seek times (where the access arm is moved a small amount -- only to the next cylinder), and full-stroke seek times (where the access arm moves from the very first cylinder to the very last one). For example, here are the seek times for a high-performance hard drive: