Compaq 307560-001 Disk Subsystem Performance and Scalability - Page 7

ECG025.0997 WHITE PAPER (cont.) ... Average Latency Manufacturers have built and continue to build hard disks that spin at designated rates. In the early years of the personal computer (PC) industry, hard disks on the market could spin at approximately 3600 RPMs. As the market demand for better system performance increased, disk manufacturers responded by supplying faster spin rates for hard disks. By producing faster spinning disks, manufacturers reduced the amount of overall access time. Average latency directly correlates to the spin rate of the disk drive because it is, as defined earlier in Table 2, the time required for the disk to spin half a revolution. Therefore, this direct relationship in improving hard disk spin rates can contribute to better system performance by reducing the average latency on a disk. Manufacturers understand the need for better system performance and continue to provide new and improved hard disks. With today's hard disks spinning at 7200 revolutions per minute (RPMs) and the hard disks of tomorrow spinning at the rate of 10,000 RPMs, we can see that manufactures continue to address the issue of faster performance. Table 3 provides a brief history on hard disks listing spin rates, disk capacities available and approximate dates the disks were available to the market. Disk Spin Rate 3600 RPMs 4500 RPMs 5400 RPMs 7200 RPMs 10,000 RPMs Table 3: Hard Disk History Disk Capacity Up to 500 MB 500 MB - 4.3 GB 500 MB - 6 GB 1 GB - 9.1 GB 4.3 GB and 9.1 GB Approximate Date Used 1983 - 1991 1991 - Present 1992 - Present 1993 - Present 1997 - Present Now that we discussed the direct relationship between disk spin rates and system performance, let's examine how drive scaling can affect latency. In Figure 2 - Example 1, the initial configuration shows the disk has to spin halfway around before the disk head can start to retrieve data from sector 5. In the new configuration, the disk has to spin half the distance than before to retrieve the same data. Thus, the latency time has been cut in half. 7