The answer is the /var partition. This is the correct choice because the 'no space left on device' error during a commit in Junos typically means the system cannot write configuration files to directories like /var/tmp or /var/db/config, which reside under /var. The /var filesystem is the primary location for accumulating log files, core dumps, and operational data, making it the most common partition to fill up over time. On the JNCIA-Junos exam, this question tests your understanding of Junos filesystem roles and common troubleshooting scenarios—a frequent trap is assuming the root (/) partition is full, but /var is the usual culprit due to unchecked log growth. A solid memory tip is to think of /var as the "variable" storage that varies in size as logs and cores pile up, so when you see a commit failure, always check /var first.
JNCIA-JUNOS Operational Monitoring and Maintenance Practice Question
This JNCIA-JUNOS practice question tests your understanding of operational monitoring and maintenance. 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.
Exhibit
Refer to the exhibit.
user@router> show system storage
Filesystem Size Used Avail Capacity Mounted on
/dev/da0s1a 1.5G 1.2G 300M 79% /config
/dev/da0s1f 1.5G 1.5G 0M 100% /var
/dev/da0s1e 1.5G 500M 1.0G 33% /tmp
tmpfs 256M 1.0M 255M 0% /dev
Refer to the exhibit. An engineer is trying to commit a configuration but receives a 'no space left on device' error. Which filesystem is most likely full?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
/var
The /var filesystem on Junos stores log files, core dumps, and other operational data. When the device runs out of disk space, it is most commonly the /var partition that is full, as it accumulates logs and crash files over time. The 'no space left on device' error during a commit indicates that the system cannot write the new configuration to the /var/tmp or /var/db/config directory, which reside under /var.
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.
✗
/config
Why it's wrong here
/config is at 79% capacity, not full.
✗
/tmp
Why it's wrong here
/tmp has 33% usage, space available.
✗
/dev
Why it's wrong here
/dev is tmpfs with 0% usage.
✓
/var
Why this is correct
/var is at 100% capacity, causing no space left error.
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 often assume the /config filesystem is the one that stores configurations and thus must be full, but in Junos, the commit process uses /var/tmp as a staging area, making /var the most likely culprit when a commit fails due to disk space.
Detailed technical explanation
How to think about this question
Under the hood, Junos uses a separate /var partition (often a dedicated slice or logical volume) to isolate operational data from the system image and configuration. The commit process writes the new configuration to /var/tmp before validating and copying it to /config, so a full /var directly prevents commits. In real-world scenarios, log rotation (via 'set system syslog archive size') and core dump management (via 'set system dump-device') are critical to prevent /var from filling up, especially on devices with limited flash storage.
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 JNCIA-JUNOS 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.
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: /var — The /var filesystem on Junos stores log files, core dumps, and other operational data. When the device runs out of disk space, it is most commonly the /var partition that is full, as it accumulates logs and crash files over time. The 'no space left on device' error during a commit indicates that the system cannot write the new configuration to the /var/tmp or /var/db/config directory, which reside under /var.
What should I do if I get this JNCIA-JUNOS 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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