On a busy system ii can lead to wild thrashing of the disk heads as different

On a busy system ii can lead to wild thrashing of the

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basis. On a busy system, ii) can lead to wild thrashing of the disk heads as different processes first try to move them one way and then another. The AmigaDOS system (at least up to 1.3) suffered from this problem even if there were only two processes. The system tried to time-share the disk which resulted in a more than fifty percent loss in performance. The user could wait for minutes while the system tried to thrash out a job which could have taken seconds if one job had been allowed to complete first without interruption. 5.3.4.2 SSTF - Shortest seek time first To get the fastest response (ignoring mechanical restrictions) we could try to sort disk requests according to those which will cause the smallest movements of the disk head. Again, this does not protect the head from direction reversals, only from large movements. Also, like all priority scheduling, it could lead to starvation of some requests. 5.3.4.3 SCAN, C-SCAN and LOOK The scanning method is to order requests so that we only move the disk head in one direction at a time. Since the disk heads only move if we need to change tracks, all requests are ordered according to which track they lie on. The heads start at the first track and move uni-directionally to the next request and then the next etc. When they reach the inside of the disk, they reverse direction and come back again. This is also called the elevator or lift algorithm since this is the way many elevators are scheduled. The C-SCAN or circular scan is a slight variant: when the heads hit the end track, they come immediately back to the beginning and start again, so that they always move in the same direction. Of course, neither algorithm needs to go as far as the last track if there are no requests for data there. The LOOK algorithm is the same as SCAN or C-SCAN but does not move into areas of the disk where no requests are waiting. Figure 5.9: Scanning disk scheduling algorithms. 5.3.4.4 Which method? The choice of scheduling algorithm depends on the nature of disk usage. For heavily use disks the SCAN / LOOK algorithms are well suited because they take care of the hardware and access requests in a reasonable order. There is no real danger of starvation, especially in the C- SCAN case. The arrangement of data on a disk play an important role in deciding the efficiency of data-retrieval. In the next section we shall look at the high-level structures which the operating system places on top of sectors. This determines the level at which we are most used to seeing the disk.
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5.3.5 Partitions For the purposes of isolating special areas of the disk, most operating systems allow the disk surface to be divided into partitions . A partition (also called a cylinder group ) is just that: a group a cylinders which lie next to each other. By defining partitions we divide up the storage of data to special areas, for convenience.
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