LFCS Operation of Running Systems Practice Question
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?
⚠ Common exam trap
It's easy for candidates to 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.
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.
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
High await with low svctm points to I/O queueing, not CPU contention; CPU saturation would show high load or run-queue delays, not disk wait. It is tempting because slow peak-hour performance often stems from CPU, and would be correct if vmstat showed runnable processes waiting.
- ✗
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 would raise svctm, not leave it at 5 ms while await exceeds 1000 ms; the gap indicates queueing. It is tempting because disk faults cause slowness, and would be correct if iostat showed elevated service times or errors.
- ✓
There is a large queue of I/O requests waiting to be serviced.
Why this is correct
Await of 1000ms against svctm of 5ms means requests spend almost all their time queued rather than being serviced. The disk itself is fast; the bottleneck is the backlog of pending I/O requests exceeding what sda can dispatch concurrently.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.