Oracle Tips by Burleson

Disk I/O Tuning

Chapter 1: General Disk Architecture

Disk architecture hasn't changed in the general sense in the last seventeen or so years. Disks have gotten smaller and faster but their general features have not changed that much. Disks are physical entities consisting of some dimensionally stable substrate overlayed (usually aluminum alloys but now magnesium and even glass or ceramics are being considered) with a thin layer of material that can take and hold a magnetic charge. Optical media uses laser gouged pits to indicate zero or one rather than the presence or lack of magnetic signals; the mechanical technology is essentially the same in either magnetic or optical media. This general magnetic disk structure is shown in figure 1-1.

Figure 1-1: General magnetic disk structure

 

For a magnetic disk an electromagnet is used to place or remove magnetic charges to the magnetically sensitive coating. These magnetic charges are later read by a read head that senses the magnetic charge. Naturally the closer to the disk that the head fly the weaker, and thus more dense, the signals can be.