Lecture 13 Device Management

Disk -- Overview

Disk working mechanism and performance parameters

Disk -- Overview

Disk I/O transfer time

Disk -- Overview

Disk I/O transfer time

Disk -- Overview

Disk I/O transfer time

Disk -- Overview

Disk I/O transfer time

Disk --Disk Scheduling Algorithm

Improve disk access performance by optimizing the disk access request order.

• Seek time is the most time-consuming part of disk access.
• Multiple I/O requests are present on the same disk simultaneously.
• Randomly processing disk access requests performs poorly.

Disk --Disk Scheduling Algorithms - FIFO

disk --disk scheduling algorithm -- FIFO

• First in first out (FIFO) algorithm
• Process requests in order.
• Fair treatment to all processes
• Performs close to random scheduling when many processes are present.

Disk -- Disk Scheduling Algorithm -- Shortest Service Time First (SSTF)

• Selects the I/O request that requires the least movement from the current position of the disk arm.
• Always chooses the shortest seek time.
  

Disk --Disk Scheduling Algorithms - SCAN

Disk --Disk Scheduling Algorithms - SCAN

• Disk arm moves in one direction, accessing all unfinished requests.
• Continues until the arm reaches the last track in that direction, then changes direction.
• Also known as the elevator algorithm.

Disk --Disk Scheduling Algorithm -- Circular SCAN (C-SCAN)

• Limits scanning to only one direction.
• After accessing the last track, the arm returns to the other end of the disk to start the C-LOOK algorithm.
• The arm reaches the last request in that direction, then immediately reverses instead of servicing all requests on the path to the last point.

Disk --Disk Scheduling Algorithm -- Circular SCAN (N-step-SCAN)

• Arm Stickiness phenomenon
  • In algorithms like SSTF, SCAN, and C-SCAN, the disk arm may remain stationary at a certain position.
• N-step scanning algorithm
  • Divides the disk request queue into sub-queues of length N.
  • Processes each sub-queue using the FIFO algorithm.
  • The scanning algorithm handles each queue.

Disk --Disk Scheduling Algorithm -- Double Queue Scanning Algorithm (FSCAN)

FSCAN algorithm

• Divide disk I/O requests into two queues
• Alternately use the scanning algorithm to process a queue
• Newly generated disk I/O requests are placed in the other queue
• All new requests are deferred until the next scan

FSCAN is a simplification of the N-step scan algorithm

FSCAN only splits the disk request queue into two subqueues