Disk I/O: Consider a magnetic disk with N cylinders rotating at a speed of R revolutions per minute. The sector size on the disk is constant. Requests for sectors arriving from the file system can be located anywhere on the disk with equal probability and the locations of the requested sectors are independent of each other. Under these assumptions, on average, over how many disk cylinders will the head have to seek to get from one requested sector to the next? • On average, once the head is positioned over the right cylinder, how long will it take before the head is positioned at the beginning of the right sector? • Say the disk (one platter) has 1000 cylinders and a sector size of 1024 bytes. The transfer time per sector is 0.1 milliseconds (i.e., the raw data rate is 10 Megabytes per second). The rotation speed is 7200 revolutions per minute. The time to seek over X cylinders is given by the formula 5 + X/100 milliseconds. Under the same assumptions above, what will the maximum effective data rate be? What percentage is this of the “raw” data rate? What effect would changing the sector size to 4 Kilobytes have?
Disk I/O: Consider a magnetic disk with N cylinders rotating at a speed of R revolutions per minute.
The sector size on the disk is constant. Requests for sectors arriving from the file system can be located anywhere on
the disk with equal probability and the locations of the requested sectors are independent of each other.
Under these assumptions, on average, over how many disk cylinders will the head have to seek to get from
one requested sector to the next?
• On average, once the head is positioned over the right cylinder, how long will it take before the head is
positioned at the beginning of the right sector?
• Say the disk (one platter) has 1000 cylinders and a sector size of 1024 bytes. The transfer time per sector is
0.1 milliseconds (i.e., the raw data rate is 10 Megabytes per second). The rotation speed is 7200 revolutions per minute.
The time to seek over X cylinders is given by the formula 5 + X/100 milliseconds. Under the same assumptions above,
what will the maximum effective data rate be? What percentage is this of the “raw” data rate? What effect would
changing the sector size to 4 Kilobytes have?
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