Operating Systems - Disk Scheduling - SSTF, SCAN, C-SCAN
If the disk head is at track 30 and pending requests are at tracks 10, 20, 40, and 50, which request will SSTF service first and why?
If the disk head is at track 30 and pending requests are at tracks 10, 20, 40, and 50, which request will SSTF service first and why?
easy🧠🧾 Concept Trace Q3 of Q15
Step-by-Step Solution
Solution:
Step 1: Calculate distances from current head (30)
Distances: 10 is 20 away, 20 is 10 away, 40 is 10 away, 50 is 20 away.Step 2: Identify closest request
Tracks 20 and 40 are both 10 away, but SSTF picks the closest; tie-break usually favors lower track or earliest request.Step 3: Choose track 20
Track 20 is closest and typically chosen first due to tie-break or arrival order.Final Answer:
Option B -> Option BQuick Check:
SSTF picks closest request, here track 20 [OK]
Quick Trick: SSTF picks request with minimum seek distance [OK]
Common Mistakes:
MISTAKES- Picking lowest track number regardless of distance
- Assuming ascending order servicing
Trap Explanation:
PITFALL- Candidates confuse SSTF with FCFS or SCAN, ignoring actual seek distance.
Interviewer Note:
CONTEXT- Tests understanding of SSTF's core selection principle.
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