Disk Scheduling

Disk Scheduling Metrics

Metrics used to evaluate disk performance:

• Seek Time: Time taken to locate the disk arm to a specified track.
• Rotational Latency: Time taken by the desired sector of disk to rotate into a position to access read/write heads.
• Transfer Time: Time to transfer the data. Depends on rotating speed and number of bytes.
• Disk Access Time: Seek Time + Rotational Latency + Transfer Time.
• Disk Response Time: Average of time spent by a request waiting to perform its I/O operation.

Disk Scheduling Algorithms

Algorithms used to schedule I/O requests arriving for disk:

• FCFS: Requests are addressed in the order they arrive in the disk queue.
• SSTF (Shortest Seek Time First): Requests having the shortest seek time are executed first.
• SCAN (Elevator Algorithm): Disk arm moves in a particular direction and services requests in its path, then reverses direction at the end.
• CSCAN: Disk arm scans the path, then reverses and scans the same path again. Prevents waiting at the other end.
• LOOK: Similar to SCAN but goes only to the last request to be serviced before reversing.
• CLOOK: Similar to CSCAN but goes only to the last request before reversing.