Disk Scheduling Algorithms

FCFS (First Come First Serve)

SSTS (Shortest Seek Time First)

• Selects the request with the minimum seek time from the current head position.
• May cause starvation

SCAN

• Disk arm starts at one end of the disk, moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.

C-SCAN

• More uniform wait time than SCAN
• Head moves from one end of the disk to the other, servicing requests as it goes.
• When it reaches the other end, it immediately returns to the beginning of the disk—does not service requests on the return trip

LOOK

• Version of SCAN
• Arm only goes as far as the last request in each direction, then reverses direction immediately without going all the way to the end of the disk.

C-LOOK

• A version of C-SCAN
• Arm only goes as far as the last request in one direction, then reverses direction immediately without first going all the way to the end of the disk.