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Thursday, January 28, 2010

C-SCAN Scheduling and LOOK Scheduling

Circular SCAN is a variant of SCAN that is designed to provide a more uniform wait time. Like SCAN, C-SCAN moves the head from one end of the disk to the other, servicing requests along the way. When the head reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip. The C-SCAN scheduling algorithm essentially treats the cylinders as a circular list that wraps around from the final cylinder to the first one.

Both SCAN and C-SCAN move the disk arm across the full width of the disk. In practice, neither of the algorithm is implemented this way. More commonly, the arm goes only as far as the final request in each direction. Then, it reverses the direction immediately, without first going all the way to the end of the disk. These versions of SCAN and C-SCAN are called LOOK and C-LOOK scheduling.

How to select a Disk Scheduling algorithm ?
With any scheduling algorithm, performance depends heavily on the number and types of requests. Suppose that the queue has just one request, then, all the scheduling algorithms are forced to behave the same, because they have no choice except one. They all behave like FCFS scheduling.
The requests for disk service can be greatly influenced by the file allocation method. The location of directories and index blocks is also important. Since every file must be opened to be used, and opening a file requires searching the directory structure, the directories will be accessed frequently.
The disk scheduling algorithm should be written as a separate module so that it can be replaced with a different algorithm if necessary. Either SSTF or LOOK is a reasonable choice for default algorithm.

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