Sequential access and ZCAV
In many database situations, what you're also concerned about is the streaming sequential read or write rate of the drive, where it's just staying in one area instead of seeking around. Computing this value is complicated by the nature of how disks are built.
The first thing to realize about modern hard disks is that the speed you'll see from them depends highly on what part of the disk you're reading from. Disks spin at one speed all of the time, referred to as Constant Angular Velocity or CAV. A typical drive nowadays will spin at 7200 RPM, and the actual disk platter is circular. When the disk read/write head is near the outside of the disk, the speed of the part passing underneath it is faster than on the inside. This is the same way that in a car, the outside edge of a tire travels further than the inside one, even though the actual rotation count is the same.
Because of this speed difference, manufacturers are able to pack more data...
