disk sheduling

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Chapter : Mass-Storage Systems


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Overview of Mass Storage Structure

Magnetic disks provide bulk of secondary storage of modern

computers Drives rotate at 60 to 200 times per second

Transfer rateis rate at which data flows between drive andcomputer

Positioning time(random-access time) is time to move disk

arm to desired cylinder (seek time) and time for desired sectorto rotate under the disk head (rotational latency)

Head crashresults from disk head making contact with the disksurface

Thats bad

Disks can be removable

Drive attached to computer via I/O bus


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Moving-head Disk Machanism


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Disk Scheduling (Cont.)

Several algorithms exist to schedule the servicing of disk I/Orequests.

We illustrate them with a request queue (0-199).

98, 183, 37, 122, 14, 124, 65, 67

Head pointer 53


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FCFS

Illustration shows total head movement of 640 cylinders.


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SSTF

Selects the request with the minimum seek time from the currenthead position.

SSTF scheduling is a form of SJF scheduling; may causestarvation of some requests.



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SSTF (Cont.)


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SCAN

The disk arm starts at one end of the disk, and moves toward theother end, servicing requests until it gets to the other end of thedisk, where the head movement is reversed and servicingcontinues.

Sometimes called the elevator algorithm.



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SCAN (Cont.)


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C-SCAN

Provides a more uniform wait time than SCAN.

The head moves from one end of the disk to theother servicing requests as it goes. When itreaches the other end, however, it immediately

returns to the beginning of the disk, withoutservicing any requests on the return trip.


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C-SCAN (Cont.)


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C-LOOK

Version of C-SCAN

Arm only goes as far as the last request in eachdirection, then reverses direction immediately,without first going all the way to the end of the

disk.


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C-LOOK (Cont.)


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End of Chapter 12

disk sheduling

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