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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk Internal Performance Factors | Mechanical Design Factors ]

Size and Number of Platter Surfaces

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

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