Evaluating Disks

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Having a good grasp of the different types of drives allows a user to make a better decision regarding which disk to buy for a particular application. All of the above data factors into this decision, but such choices basically boil down to the elements of speed, capacity, and reliability. Speed consists of seek time and rotational latency. As discussed previously, seek time is how long it takes for the read/write head to move to the correct track. A lighter head/actuator assembly, greater areal density, and smaller disk size all contribute to reducing seek time. Rotational latency is how long it takes for the disk to then spin to the spot where the desired information is stored, so it basically is a function of the disk revolutions per minute. Drive software can also improve the data access speed by caching the I/O requests and performing them in an order that reduces head movement. Keeping files defragmented, however, can have a greater positive impact than any of these factors. Even if, for example, the disk revolutions per minute are tripled, such an improvement does not begin to compensate for having to move the head to dozens or even thousands of locations to retrieve all the bits of the file. What about disk capacity? Capacity depends on areal density and platter size. The actual usable capacity can be reduced, however, by using incorrect cluster sizes, as discussed.

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