Introduction to Hard Disk Scheduling - FCFS Scheduling

 

Introduction to Hard Disk Scheduling

🔷 What is Disk Scheduling?

Hard Disk Scheduling is a technique used by the operating system to decide the order in which disk I/O requests are serviced.

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🔷 Why is Disk Scheduling Needed?

In a multiprogramming system:

• Multiple processes generate I/O requests simultaneously

• These requests are placed in a disk queue

👉 The OS must decide:

• Which request to serve first

• In what order to minimize delay

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🔷 Goals of Disk Scheduling

• ⚡ Minimize access time

  • Reduce seek time (head movement)

  • Reduce rotational latency

• 📈 Maximize bandwidth

  • Transfer more data in less time

• ⚖️ Ensure fairness

  • Avoid starvation of requests

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🔷 Key Insight

👉 Disk performance depends heavily on:

• Order of servicing requests

• Because head movement is costly

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FCFS (First-Come, First-Served) Scheduling

🔷 What is FCFS?

FCFS scheduling services disk requests in the order they arrive.

👉 Also called:

• FIFO (First-In, First-Out)

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🔷 How It Works

• Requests are placed in a queue

• OS processes them sequentially

• No reordering is done

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🔷 Example

Queue:

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

Initial head position:

53

Head movement sequence:

53 → 98 → 183 → 37 → 122 → 14 → 124 → 65 → 67

👉 Total movement = 640 cylinders

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⚠️ Problem with FCFS

🔴 High Seek Time

• Disk head moves back and forth unnecessarily

• Example:

  • 122 → 14 → 124 (large jumps)

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🔴 Poor Performance

• Does not optimize head movement

• Leads to:

  • Longer access time

  • Lower efficiency

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✅ Advantage

• ✔️ Simple to implement

• ✔️ Fair (no starvation)

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🎯 Key Takeaways

• Disk scheduling improves performance of HDDs

• FCFS is:

  • Simple and fair

  • But inefficient due to excessive head movement

• Better algorithms aim to reduce seek time