PC Hardware in a Nutshell, 3rd Edition

Average Access

Although it bears superficial resemblance to the hard drive rating with the same name, average access time for a CLV CD-ROM drive is much more complex to calculate, and is subject to manipulation by drive manufacturers who wish to boost their performance figures. Hard drives spin at a constant rate, and average access time is calculated as average seek time (the time required for the heads to move over the proper track) plus latency (the time required for the disk to spin the one-half revolution required on average to move the correct sector under the heads).

Average access for CLV CD-ROM drives was originally calculated using a similar 1/3 stroke method, assuming that the drive would be used mainly for reading large multimedia files. In about 1993, some manufacturers began substituting “random access” for 1/3 stroke testing. This method was subject to abuse because manufacturers could define the size of the zone they used for testing. Some chose very small zones to boost their average access ratings, with the result that some drives were advertised with average access times of less than 60 ms. Worse still, some makers began promoting seek time as a performance measure. Seek time is a useless performance measure for a CLV drive because it ignores the fact that a CLV drive needs to speed up or slow down the disc to the speed required for data to be read. The time required for this step—roughly analogous to latency, but subject to much wider variation—is determined ...