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| Table 9.4 Typical drive characteristics. |
| | | | DEC RK81 | | No. of cylinders/device | 1787 cylinders | | 1258 | | No. of tracks/cylinder | 16 tracks | | 14 | | No. of sectos/track | 77 sectors | | 52 | | Capacity/sector | 512 bytes | 512 | 512 | | Capacity | 1 GB | 7.5 GB | 0.5 GB | | Access time | 15 ms | 24 ms | 16.6 ms |
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We will use a typical sector size of 512 bytes. Until quite recently, drives rotated at 3600 rpm and had a total rotational delay of 16.6 milliseconds and an expected rotational delay of half that, or 8.3 milliseconds. This assumes that the starting address of a block is uniformly distributed over a track. Currently available drives rotate at 5400 to 7900 rpm. |
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Seek time is more varied and complex. It is a function of the file size, the cylinder size, and the characteristics of the motor that is used to actuate the arm to move to the designated cylinder. Generally, seek time is a function of the number of tracks transited by the arm. Asymptotically, if the requests for cylinder access were uniformly distributed, we would expect that the average distance that the arm would have to travel would be n/3 cylinders, where n is the number of cylinders that contain user files. Several things make this an inaccurate assumption: |
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1. It has been observed that about half of all seeks are zero-distance. |
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2. Seek time is not linearly proportional to the distance the access arm travels, due to arm starting and stopping inertia. |
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Bitton [39] and Scranton [255] have shown that the arm movement for a high-performance voice coil motor can be approximated as: |
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Including the effect of zero-distance seeks mentioned before produces an estimate of expected seek time: |
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Table 9.5 indicates some estimates of a and b parameters for various commercial drives. |
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9.4 Simple I/O Transactions |
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Suppose we have a processor with locally attached disk. The processor executes in the user state for a while, then makes an I/O (disk) request. |
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