Operating Systems - Disk Scheduling - SSTF, SCAN, C-SCAN
Given a disk head currently at track 50 with pending requests at tracks 45, 55, 60, and 40, which algorithm would service the requests in the order 45, 40, 55, 60?
Given a disk head currently at track 50 with pending requests at tracks 45, 55, 60, and 40, which algorithm would service the requests in the order 45, 40, 55, 60?
easy🔍 Pattern Recognition Q2 of Q15
Step-by-Step Solution
Solution:
Step 1: Analyze the order given
The requests are serviced first towards lower tracks (45, 40), then higher (55, 60).Step 2: Match with algorithm behavior
SCAN moves the head in one direction servicing all requests before reversing, matching this order.Step 3: Check SSTF and C-SCAN
SSTF would pick 45, then 55 (closest next), not 40 after 45; C-SCAN moves in one direction only, so order differs.Final Answer:
Option C -> Option CQuick Check:
SCAN moves down servicing 45 and 40, then reverses up [OK]
Quick Trick: SCAN services requests in one direction then reverses [OK]
Common Mistakes:
MISTAKES- Assuming SSTF always picks closest globally
- Confusing C-SCAN direction
Trap Explanation:
PITFALL- Candidates often confuse SSTF's closest-next with SCAN's directional servicing, leading to wrong order assumptions.
Interviewer Note:
CONTEXT- Checks if candidate can identify algorithm behavior from request servicing order.
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