- A
The CPU is overloaded, causing processes to wait for CPU time.
Why wrong: CPU overload would show high CPU utilization and possibly high run queue, but the iostat output shows high I/O wait, not CPU wait.
- B
The system is using swap space heavily, causing disk I/O.
Why wrong: Heavy swap usage would cause high disk utilization, but the svctm would likely be higher because swapping involves random I/O, and the await would be high as well. However, the low svctm suggests the disk is serving requests quickly once they start, which is not typical of swap.
- C
The disk is experiencing hardware errors.
Why wrong: Hardware errors typically cause high svctm (due to retries) and may show errors in dmesg, not low svctm with high await.
- D
There is a large queue of I/O requests waiting to be serviced.
A high await with low svctm indicates that the disk is fast but there are many requests queued, so each request spends a long time waiting before being serviced.
Quick Answer
The answer is a large queue of I/O requests waiting to be serviced. This conclusion follows directly from comparing await and svctm: await measures the total time from request submission to completion, including queue wait time, while svctm measures only the actual disk service time. With await exceeding 1000 ms and svctm at just 5 ms, the vast majority of latency is spent waiting in line, not being processed—a classic sign of high I/O queue depth. On the LFCS exam, this scenario tests your ability to distinguish between queue congestion and slow hardware; a common trap is to blame the disk itself when svctm is low. Remember the mnemonic: await includes the wait, svctm is just the service—if await dwarfs svctm, the queue is the problem.
LFCS Operation of Running Systems Practice Question
This LFCS practice question tests your understanding of operation of running systems. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An administrator is troubleshooting a server that runs a critical application. The server has 16 GB of RAM and 8 CPU cores. The administrator notices that the server becomes very slow during peak hours. Analysis of 'iostat -x 1' shows that the average wait time (await) for the main disk (sda) is consistently above 1000 ms, while the average service time (svctm) is around 5 ms. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
There is a large queue of I/O requests waiting to be serviced.
The 'await' value in iostat represents the average time (in milliseconds) for I/O requests to be serviced, including time spent waiting in the queue. With 'await' at 1000+ ms and 'svctm' at only 5 ms, the vast majority of the time is spent waiting, not being serviced. This indicates a large queue of pending I/O requests, which is the direct cause of the slowdown.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The CPU is overloaded, causing processes to wait for CPU time.
Why it's wrong here
CPU overload would show high CPU utilization and possibly high run queue, but the iostat output shows high I/O wait, not CPU wait.
- ✗
The system is using swap space heavily, causing disk I/O.
Why it's wrong here
Heavy swap usage would cause high disk utilization, but the svctm would likely be higher because swapping involves random I/O, and the await would be high as well. However, the low svctm suggests the disk is serving requests quickly once they start, which is not typical of swap.
- ✗
The disk is experiencing hardware errors.
Why it's wrong here
Hardware errors typically cause high svctm (due to retries) and may show errors in dmesg, not low svctm with high await.
- ✓
There is a large queue of I/O requests waiting to be serviced.
Why this is correct
A high await with low svctm indicates that the disk is fast but there are many requests queued, so each request spends a long time waiting before being serviced.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse 'await' with 'svctm' or assume high 'await' always means slow disk hardware, when in fact the low 'svctm' proves the disk is fast but overwhelmed by queue depth.
Trap categories for this question
Command / output trap
CPU overload would show high CPU utilization and possibly high run queue, but the iostat output shows high I/O wait, not CPU wait.
Detailed technical explanation
How to think about this question
The 'await' metric includes both queueing time and service time, while 'svctm' is the average time for the disk to actually complete a request. A high 'await' with low 'svctm' is a classic sign of a saturated I/O subsystem where requests are piling up faster than the disk can process them, often due to excessive writes, a misconfigured I/O scheduler (e.g., CFQ vs deadline), or a single-threaded application bottleneck. In real-world scenarios, this can occur with database servers under heavy write load or when using a single spindle for a high-traffic log partition.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Operation of Running Systems — study guide chapter
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FAQ
Questions learners often ask
What does this LFCS question test?
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: There is a large queue of I/O requests waiting to be serviced. — The 'await' value in iostat represents the average time (in milliseconds) for I/O requests to be serviced, including time spent waiting in the queue. With 'await' at 1000+ ms and 'svctm' at only 5 ms, the vast majority of the time is spent waiting, not being serviced. This indicates a large queue of pending I/O requests, which is the direct cause of the slowdown.
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 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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