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Speed Change Time
One performance factor that is relevant for (most) CD-ROM drives but not for hard disks is speed change time. Standard CD-ROM drives use constant linear velocity (CLV) reading, which means that the same amount of data is read from the disk in each unit of time. This means the speed of the disk must change as the head is moved from the inner part of the disk to the outer, or vice-versa. This is described in detail here.
It can take a significant amount of time to change the speed of the spindle motor, especially with faster drives. The difference in speed can be several thousand RPM in the faster drives, between the speed at the inside and the outside of the disk. When the disk is doing a lot of random accesses, you can sometimes hear this as the whirring noise from the motor audibly "ramps up" in frequency and "ramps down" as it moves around the disk.
The delay in changing speeds can slow down random accesses on the disk, sometimes significantly. This metric is not usually specified directly by the manufacturer, but instead is usually lumped in to the access time measurement. Changing the speed of the disk can be done in parallel with (at the same time as) the head is moved during a seek. If the seek is completed, however, and the drive hasn't finished changing speeds yet, there will be a small delay.
Due to the amount of time it takes to change speeds (along with other reasons), many newer drives have changed to a constant angular velocity system (CAV). These drives do not change speeds, and therefore do not incur the speed change penalty as they move over the disk. However, they give up some transfer rate performance as they move to the inside of the disk, since they are covering less linear distance in each unit of time. Since these disks are usually quite fast to begin with, this is often a good tradeoff. A 24X CD-ROM drive is only 24X at the outer edge of the disk, but it is still the equivalent of at least 12X even on the inside of the disk. These drives are often more pleasant to use because there is less noise from the drive changing speeds all the time.
Next: Seek Time