The data in the hard disk is stored on the platter surfaces. (The operation and
characteristics of the platters and media are described in detail here, including a lot of performance-relevant detail.)
The number of platters and the size of the platters themselves vary between different hard
disk designs, and have an important impact on performance in several ways.
First, let's look at platter size. As discussed in much detail here, the trend is towards smaller and smaller
platter sizes for a number of reasons; two of them being particularly important to
performance. The first one is that smaller platters allow the data on the drive to be
located physically closer together, so there is less distance for the hard disk actuator to have to move when doing random reads or writes on
the disk. This directly improves positioning performance on random accesses. The second is
that smaller platters have lower mass and higher rigidity, which enables them to be spun
at higher speeds for a given power of spindle motor (or conversely, to use a lower-powered
spindle motor for the same spin speed). The main cost of using smaller platters is reduced
capacity, but with areal density constantly
increasing--thus doubling capacity per square inch every year or two anyway--this is a
trade-off more people than ever are willing to make.
The number of platters has a more subtle
influence on performance; this is why you will sometimes see small differences in the
specifications of drives of different capacity in the same model family. The first impact
is a relatively simple: more platters means more weight and thus more for the spindle
motor to turn. This generally means that the spin-up speed and power consumption of a
drive with four platters will be a little higher than those figures for the same drive
with two platters.
The other impact of the number of platters is a bit more controversial: not everyone
agrees on the extent to which these effects exist. All else being equal, a drive with more
platters will have slightly better positioning performance and a slightly higher
sustained transfer rate than one with fewer platters. If you double the number of data
storage surfaces, you can store the same amount of data in (roughly) half as many
cylinders; this keeps the data "closer together" physically on the drive,
reducing the extent to which the actuator must move when doing seeks. You also replace
many cylinder switches with head switches when you have more platters; a
one-platter drive will have a 1:1 ratio of head switches to cylinder switches on a
sustained read; a four-platter drive will have a 7:1 ratio. Head switches are faster than
cylinder switches, so this slightly improves STR, though it's certainly not a large
effect. I show the difference between drives of the same family in the discussion of the sustained transfer rate specification.
The size and number of platter surfaces on the drive have an impact on seek time (and hence access
time), media transfer rate and sustained transfer rate, spin-up speed, and power
consumption. Of course, the basic design of the drive also matches the platter size
and number to the power of the spindle motor.
Next: Actuator Characteristics
