Quick Answer
The answer is the root filesystem, specifically /dev/sda1 mounted on /. When the root filesystem reaches 100% capacity, services cannot write log files because critical directories like /var/log reside under the root partition unless /var is mounted separately. Log writes fail because there is zero free space for new file creation or appending, and the kernel’s filesystem operations halt when no inodes or blocks remain. On the CompTIA Linux+ XK0-005 exam, this scenario tests your ability to interpret df -h output and identify that a full root partition—not a separate /var or /home—is the culprit. A common trap is assuming a separate /var mount would be the issue, but the exhibit shows /dev/sda1 at 100% usage with no independent /var partition. Memory tip: “Root full? Logs null.” If the root filesystem is maxed out, any service trying to write to /var/log will fail, so always check the mount point for / first.
This XK0-005 practice question tests your understanding of troubleshooting. 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.
Refer to the exhibit. Filesystem Size Used Avail Use% Mounted on /dev/sda1 20G 19G 1.0G 95% / /dev/sda2 50G 30G 20G 60% /var /dev/sda3 30G 10G 20G 33% /home
Refer to the exhibit. An administrator is troubleshooting an issue where services cannot write log files. Based on the output, which filesystem is most likely the 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.
Refer to the exhibit. Filesystem Size Used Avail Use% Mounted on /dev/sda1 20G 19G 1.0G 95% / /dev/sda2 50G 30G 20G 60% /var /dev/sda3 30G 10G 20G 33% /home
/dev/sda3 (/home)
Why wrong: Has plenty of space.
Swap partition
Why wrong: Not shown and not directly related to log writes.
/dev/sda2 (/var)
Why wrong: Still has free space.
/dev/sda1 (/)
Root at 95% is nearly full.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
/dev/sda1 (/)
The output shows that the root filesystem /dev/sda1 mounted on / is at 100% usage. Since log files are typically written under /var/log, which resides on the root partition unless /var is a separate mount point, a full root filesystem prevents services from writing log files. The correct answer is D because the root filesystem is full, causing the write failures.
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.
/dev/sda3 (/home)
Why it's wrong here
Has plenty of space.
Swap partition
Why it's wrong here
Not shown and not directly related to log writes.
/dev/sda2 (/var)
Why it's wrong here
Still has free space.
/dev/sda1 (/)
Why this is correct
Root at 95% is nearly full.
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
CompTIA often tests the misconception that log files always reside on a separate /var partition, but in many default configurations, /var is part of the root filesystem, so a full root partition directly impacts log writes.
Trap categories for this question
Command / output trap
Not shown and not directly related to log writes.
Detailed technical explanation
In Linux, log files are typically written to /var/log, which is part of the root filesystem unless /var is mounted as a separate partition. When the root filesystem reaches 100% capacity, the kernel returns ENOSPC errors to any write operation, including syslog, rsyslog, or systemd-journald. The 'df -h' command shows usage per mount point, and a full root partition (/) is a common cause of service failures due to log write errors.
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 XK0-005 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.
Troubleshooting — study guide chapter
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FAQ
Troubleshooting — This question tests Troubleshooting — Read the scenario before looking for a memorised answer..
The correct answer is: /dev/sda1 (/) — The output shows that the root filesystem /dev/sda1 mounted on / is at 100% usage. Since log files are typically written under /var/log, which resides on the root partition unless /var is a separate mount point, a full root filesystem prevents services from writing log files. The correct answer is D because the root filesystem is full, causing the write failures.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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