This LFCS practice question tests your understanding of operation of running systems. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A Linux server experiences intermittent high load averages but low CPU utilization. The administrator suspects an I/O bottleneck. Which command best identifies the specific device causing the bottleneck?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
sar -d 1
Why wrong: Can show disk stats but less immediate than iostat.
vmstat 1
Why wrong: Shows CPU and memory, not per-device I/O.
top -d 1
Why wrong: Shows process CPU usage, not I/O per device.
iostat -x 1
Shows extended I/O stats per device, identifying bottlenecks.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
iostat -x 1
Option D is correct because `iostat -x 1` provides extended device statistics, including `%util`, `await`, and `svctm`, which directly indicate the specific block device causing an I/O bottleneck. The `-x` flag reports per-device metrics like average I/O queue size and service time, making it the precise tool to identify a slow or overloaded disk under high load with low CPU usage.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
sar -d 1
Why it's wrong here
Can show disk stats but less immediate than iostat.
vmstat 1
Why it's wrong here
Shows CPU and memory, not per-device I/O.
top -d 1
Why it's wrong here
Shows process CPU usage, not I/O per device.
iostat -x 1
Why this is correct
Shows extended I/O stats per device, identifying bottlenecks.
Clue confirmation
The clue words "best", "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates see high `wa` in `vmstat` or `top` and assume those tools are sufficient to identify the I/O bottleneck, but they lack per-device granularity, which `iostat -x` uniquely provides.
Trap categories for this question
Command / output trap
Can show disk stats but less immediate than iostat.
Detailed technical explanation
Under the hood, `iostat -x` reads counters from `/proc/diskstats` and calculates metrics like average queue length (`avgqu-sz`) and I/O service time (`svctm`). A high `%util` (near 100%) indicates the device is saturated, meaning I/O requests are queuing, which explains high load averages despite low CPU utilization. In real-world scenarios, a misconfigured RAID controller or a failing disk can cause intermittent I/O spikes that `iostat -x` reveals through erratic `await` values.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
The correct answer is: iostat -x 1 — Option D is correct because `iostat -x 1` provides extended device statistics, including `%util`, `await`, and `svctm`, which directly indicate the specific block device causing an I/O bottleneck. The `-x` flag reports per-device metrics like average I/O queue size and service time, making it the precise tool to identify a slow or overloaded disk under high load with low CPU usage.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best", "which command". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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