Disk Seek Times

The interior of a hard disk is built from one or more circular disk platters on which data are stored. These platters spin at high speeds, and the disk heads ride over and between the platters on arms that can pivot, bringing the heads over any point on the data storage surface (see Figure 5.4). The time it takes to move the head is referred to as the seek time. In fact, there are several possible measures of seek time, such as time to seek from the center to the outer edge (or vice versa), time to seek to the mid-way point, and so on. The seek time measurement that's most often used is the average seek time, which is the average time to move the head from one location to a randomly-selected other location. Statistically speaking, this is very close to the time to seek one-third of the head's range.

Another measurement closely related to seek time involves a second component beyond the head movement: latency. Latency is the time it takes the desired sector to come up under the read/write head after a seek operation, and averages out to half the time it takes for a rotation of the disk platter. The combination of seek time and latency is known as access time.

Part II

Head arm

Head arm

Figure 5.4

Latencies involved in the movement of hard drive components are responsible for hard disk data transfer rates.

Figure 5.4

Latencies involved in the movement of hard drive components are responsible for hard disk data transfer rates.

Seek time, latency, and access time are all measured in milliseconds (ms). Hard disks in 2000 have seek times of 5ms-10ms and spin at between 5,400 and 10,000 rpm, for average latencies of 3ms-6ms (lower latencies being associated with higher spin rates). The main point to keep in mind with respect to seek time, latency, and access time is that smaller numbers are better. Keep in mind the difference between seek time and access time, too. Sometimes you'll find one vendor advertising seek time while another promotes access time, but these values are not directly comparable. If you know the spin rate of the two drives, you can convert one to the other quite easily, of course, but you must be aware of the difference in the first place.

Was this article helpful?

0 0
The Ultimate Computer Repair Guide

The Ultimate Computer Repair Guide

Read how to maintain and repair any desktop and laptop computer. This Ebook has articles with photos and videos that show detailed step by step pc repair and maintenance procedures. There are many links to online videos that explain how you can build, maintain, speed up, clean, and repair your computer yourself. Put the money that you were going to pay the PC Tech in your own pocket.

Get My Free Ebook


Post a comment