分 Suppose the hard disk with SCAN scheduling Alg has a single platter (盘面) as illustrated below , and the rotation speed for the platter is 6000 rounds per minute . The disk has only 200 tracks , and each track is composed of 100 sectors . The time cost for the disk arm to navigate between neighboring track is about 1ms. Suppose the disk arm currently hangs over the 100th track , and moves centrifugally towards lower - order tracks on the outside . The pending disk block requests are positioned on the tracks with numbers 50, 180,90,30,120, respectively , and form a queue . For each block in this queue , we do not know what are its exact sector numbers , and we can only assume that the corresponding sector randomly distributes on each track as shown below . Then , The expected time to finish the retrieval of the five sectors is1] ms . If SSTF is used , the expected time is [] ms . 0 磁道 磁头运动方向 .

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磁盘调度算法(SCAN和SSTF)

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根据题目所给的信息,我们可以画出磁道与扇区的示意图: ``` | Track 1 | | Sector 1 | Sector 2 | |---------------------| | Track 2 | | Sector 1 | Sector 2 | |---------------------| | ... | |---------------------| | Track 200 | | Sector 1 | Sector 2 | ``` 其中,每个磁道有 100 个扇区,总共有 200 个磁道。 根据题目所描述的情况,我们可以计算出使用 SCAN 调度算法时,磁头移动的距离为: - 从 100 磁道移动到 50 磁道,需要移动 50 个磁道,耗时 50ms; - 从 50 磁道移动到 30 磁道,需要移动 20 个磁道,耗时 20ms; - 从 30 磁道移动到 90 磁道,需要移动 60 个磁道,耗时 60ms; - 从 90 磁道移动到 120 磁道,需要移动 30 个磁道,耗时 30ms; - 从 120 磁道移动到 180 磁道,需要移动 60 个磁道,耗时 60ms。 因此,总共需要移动 220 个磁道,耗时为 220ms。 对于 SSTF 调度算法,我们可以按照距离当前磁头位置最近的磁道顺序来访问请求的磁道。因此,访问的顺序应该是 90、120、50、30、180。这样,总共需要移动的磁道数为 130,耗时为 130ms。 因此,答案为: - 使用 SCAN 调度算法时,耗时为 220ms; - 使用 SSTF 调度算法时,耗时为 130ms。
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