OSOperating Systems Learning StudioGuided notes + mapped practice
Question BankExam KitKnowledge Graph

Chapter 5

Disk Arm Scheduling Algorithms

Solve FCFS, SSTF, SCAN, C-SCAN, LOOK, and C-LOOK head movement problems.

Estimated time: 30 min

Disk scheduling chooses request order to reduce head movement and access delay.

These are standard numerical questions with direct marks in exam papers.

Algorithm Behaviors

From notes

Clear explanation

Each policy changes traversal pattern: queue order (FCFS), nearest neighbor (SSTF), elevator sweeps (SCAN/LOOK variants).

What it really means

You either serve in line order, nearest stop first, or route-based sweeps.

Key takeaway

Head movement pattern is the key comparison axis.

Solving Method

Added clarity

Clear explanation

Write service sequence first, then compute pairwise absolute differences and sum them.

What it really means

Sequence accuracy matters more than final arithmetic speed.

Key takeaway

Always annotate directional assumptions before calculation.

FCFS vs SSTF vs SCAN-family

Added clarity

Clear explanation

FCFS follows request arrival order, SSTF always chooses nearest pending cylinder, SCAN moves in one direction then reverses, C-SCAN returns to start and continues one-way, LOOK/C-LOOK stop at last request instead of disk end.

What it really means

It is the same request set with different travel planning strategies.

Example

From head=11 and queue [1,36,16,34,9,12], SSTF first picks 12, while FCFS first picks 1 because it arrived first.

Key takeaway

Correct service order is half of the numerical solution.

Head Movement Diagram Habit

Added clarity

Clear explanation

Plot the start head position and service order on cylinder axis, then draw arrows for every movement to avoid arithmetic misses.

What it really means

Visual path reduces mistakes better than raw subtraction only.

Example

For queue 1,36,16,34,9,12 with head 11, your path diagram quickly reveals direction choice errors in SCAN/LOOK.

Key takeaway

Draw movement diagram first, then compute totals from path.

  • - Not fixing initial direction before SCAN/LOOK
  • - Skipping movement to end tracks in SCAN/C-SCAN
  • - Arithmetic mistakes in total cylinder movement
  • - FCFS simple but often high movement
  • - SSTF greedy nearest request
  • - SCAN family sweeps by direction
  • - LOOK family turns only at last pending request
Total head movement = Sum of absolute differences between consecutive head positions

Exam lens for this topic

What evaluators usually expect in structured exam answers.

Must-use keywords

  • - Head movement
  • - Cylinder
  • - SSTF
  • - SCAN
  • - LOOK

Answer flow

  • - Write current head and pending queue
  • - State direction assumption before solving SCAN/LOOK
  • - Show service order clearly
  • - Add absolute movement values and total

Diagram expectations

  • - Head movement track diagram

Repeated pattern: Request set around cylinder 11 appears in multiple years.

Mini quiz

Quick self-check from this topic before moving ahead.

1. Which algorithm is also called elevator algorithm?

Practice Questions

  • Find total movement for FCFS and SSTF for request set around cylinder 11.

    Source: Summer 2023 Q5(C)

    Answer focus: Correct sequence and movement sum.

  • Solve LOOK and SCAN for given queue starting at cylinder 11.

    Source: Summer 2024 Q5(C) OR

    Answer focus: Direction rule and endpoint handling.

Practice from papers (end-of-topic set)

These paper questions map directly to this topic. Solve now, then compare your structure with linked topics.

Question Bank Linked Here

Open all questions

How to answer linked exam questions

Full question bank

  • Summer 2023 Q5(C)

    For requests 1,36,16,34,9,12 and current head 11 on 51-cylinder disk, calculate total movement for FCFS and SSTF.

    Answer pattern: define briefly, then solve stepwise and show final values/table.

    Disk Arm Scheduling AlgorithmsRAID and Disk Cache

  • Summer 2023 Q5(C) OR

    For requests 1,36,16,34,9,12 and current head 11 on 51-cylinder disk, calculate total movement for LOOK and SCAN.

    Answer pattern: define briefly, then solve stepwise and show final values/table.

    Disk Arm Scheduling AlgorithmsRAID and Disk Cache

  • Winter 2023 Q5(C)

    For requests 1,36,16,34,9,12 and current head 11 on 51-cylinder disk, find total movement using C-LOOK.

    Answer pattern: concept -> intuition -> steps -> concluding point with one application.

    Disk Arm Scheduling AlgorithmsRAID and Disk Cache

  • Winter 2023 Q5(C) OR

    For requests 1,36,16,34,9,12 and current head 11 on 51-cylinder disk, find total movement using C-SCAN.

    Answer pattern: concept -> intuition -> steps -> concluding point with one application.

    Disk Arm Scheduling AlgorithmsRAID and Disk Cache

  • Summer 2024 Q5(C)

    For requests 1,36,16,34,9,12 and current head 11, solve FIFO and SSTF total movement.

    Answer pattern: define briefly, then solve stepwise and show final values/table.

    Disk Arm Scheduling Algorithms

  • Summer 2024 Q5(C) OR

    For requests 1,36,16,34,9,12 and current head 11, solve LOOK and SCAN total movement.

    Answer pattern: define briefly, then solve stepwise and show final values/table.

    Disk Arm Scheduling Algorithms

Extra clarity files

These are clearly marked additions, separate from source notes.

Topic relation map

Prerequisites

Next topics

Related topics