of a disk in a computer system the time spent waiting for a disk to become free

Of a disk in a computer system the time spent waiting

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of a disk in a computer system, the time spent waiting for a disk to become free ( queuing delay ) is added to this time. Average rotation time 0.5 10000 RPM ---------------------------- 0.5 10000 60 ( ) RPS ----------------------------------------- 0.0030 sec 3.0 ms = = = =
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492 Chapter 7 Storage Systems E X A M P L E What is the average time to read or write a 512-byte sector for a disk? The advertised average seek time is 5 ms, the transfer rate is 40 MB/second, it rotates at 10000 RPM, and the controller overhead is 0.1 ms. Assume the disk is idle so that there is no queuing delay. In addition, calculate the time assuming the advertised seek time is three times longer than the measured seek time. A N S W E R Average disk access is equal to average seek time + average rotational delay + transfer time + controller overhead. Using the calculated, average seek time, the answer is 5 ms + + 0.1 ms = 5.0 + 3.0 + 0.013 + 0.1 = 8.11 ms Assuming the measured seek time is 33% of the calculated average, the answer is 1.67 ms + 3.0 ms + 0.013 ms + 0.1 ms = 4.783 ms Note that only or 0.3% of the time is the disk transferring data in this example. Even page-sized transfers often take less than 5%, so disks normally spend most of their time waiting for the head to get over the data rather than reading or writing the data. n Many disks today are shipped in disk arrays . These arrays contain dozens of disks, and may look like a single large disk to the computer. Hence, there is often another level to the storage hierarchy, the array controller . They are often key in dependability and performance of storage systems, implementing functions such as RAID (see section 7.5) and caching (see section 7.12). The Future of Magnetic Disks The disk industry has concentrated on improving the capacity of disks. Improve- ment in capacity is customarily expressed as improvement in areal density , mea- sured in bits per square inch: Through about 1988 the rate of improvement of areal density was 29% per year, thus doubling density every three years. Between then and about 1996, the rate improved to 60% per year, quadrupling density every three years and matching the traditional rate of DRAMs. From 1997 to 2001 the rate increased to 100%, or doubling every year. In 2001, the highest density in commercial products is 20 billion bits per square inch, and the lab record is 60 billion bits per square inch. Cost per gigabyte has dropped at least as fast as areal density has increased, with smaller drives playing the larger role in this improvement. Figure 7.3 on 0.5 10000 RPM ---------------------------- 0.5 KB 40.0 MB/sec ------------------------------ + 0.013 4.783 Areal density Tracks Inch --------------- on a disk surface Bits Inch ---------- on a track × =
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7.2 Types of Storage Devices 493 page 493 plots price per personal computer disk between 1983 and 2000, show- ing both the rapid drop in price and the increase in capacity. Figure 7.4 on page 494 above translates these costs into price per gigabyte, showing that it has improved by a factor of 10,000 over those 17 years. Notice the much quicker drop in prices per disk over time, reflecting faster decrease in price per gigabyte.
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