Average Latency - Compaq 108164-003 - ProLiant - 800 White Paper

ECG025.0997
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HITE APER
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Average Latency

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