Question 274 of 522

Devices, Filesystems and FHS →hardMultiple ChoiceObjective-mapped

LPIC-1 Devices, Filesystems and FHS Practice Question

This LPIC-1 practice question tests your understanding of devices, filesystems and fhs. 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.

You are a Linux administrator for a large e-commerce company. The company's application server runs Ubuntu 22.04 and uses a Btrfs filesystem for the /srv/app partition to take advantage of snapshots and compression. During a routine maintenance window, you create a snapshot of the /srv/app subvolume using 'btrfs subvolume snapshot /srv/app /srv/app_snapshot'. Later, you need to roll back to the snapshot because of a failed application update. You attempt to delete the original subvolume with 'btrfs subvolume delete /srv/app', but the command fails with the error: 'ERROR: cannot delete '/srv/app': Device or resource busy'. You check that no processes are using the directory with lsof, and it shows no open files. The /srv/app subvolume is the default subvolume and is mounted via fstab with the option 'subvol=/' (the root of the Btrfs filesystem). What is the most likely reason for the deletion failure, and how should you proceed?

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.

Question 1hardmultiple choice

Full question →

A
The subvolume is read-only; change it to read-write with 'btrfs property set -f /srv/app ro false'.
Why wrong: Read-only subvolumes can still be deleted; the error is about being busy, not read-only.
B
You cannot delete the subvolume because it is the default subvolume; you must first delete the snapshot and then recreate the original.
Why wrong: The snapshot is separate; you can delete the original if it is not the default. The problem is the default subvolume is busy because it is mounted.
C
The Btrfs kernel module has a readahead lock on the subvolume; wait for it to expire or reboot.
Why wrong: Readahead does not prevent subvolume deletion.
D
The subvolume is the default subvolume (ID 5) and cannot be deleted while it is the mounted root; use 'btrfs subvolume set-default <new_subvol_id> /srv/app', then unmount and remount with the new default subvolume, then delete the old subvolume.
The default subvolume is special and cannot be deleted while it is the mounted root. Changing the default allows deletion after remount.

Full breakdown with real-world context →

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

✓

The subvolume is the default subvolume (ID 5) and cannot be deleted while it is the mounted root; use 'btrfs subvolume set-default <new_subvol_id> /srv/app', then unmount and remount with the new default subvolume, then delete the old subvolume.

The error occurs because the subvolume at /srv/app is the default subvolume (ID 5) of the Btrfs filesystem, and it is currently mounted as the root of the filesystem via the 'subvol=/' mount option. A default subvolume cannot be deleted while it is mounted because the kernel treats it as the top-level volume. To delete it, you must first set a different subvolume as the default using 'btrfs subvolume set-default', then unmount and remount the filesystem with the new default, after which the original subvolume can be safely deleted.

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 subvolume is read-only; change it to read-write with 'btrfs property set -f /srv/app ro false'.
Why it's wrong here
Read-only subvolumes can still be deleted; the error is about being busy, not read-only.
✗
You cannot delete the subvolume because it is the default subvolume; you must first delete the snapshot and then recreate the original.
Why it's wrong here
The snapshot is separate; you can delete the original if it is not the default. The problem is the default subvolume is busy because it is mounted.
✗
The Btrfs kernel module has a readahead lock on the subvolume; wait for it to expire or reboot.
Why it's wrong here
Readahead does not prevent subvolume deletion.
✓
The subvolume is the default subvolume (ID 5) and cannot be deleted while it is the mounted root; use 'btrfs subvolume set-default <new_subvol_id> /srv/app', then unmount and remount with the new default subvolume, then delete the old subvolume.
Why this is correct
The default subvolume is special and cannot be deleted while it is the mounted root. Changing the default allows deletion after remount.
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 assume the 'Device or resource busy' error always means a process is using the directory, but in Btrfs it can also indicate that the subvolume is the mounted default, which requires changing the default before deletion.

Detailed technical explanation

How to think about this question

In Btrfs, the default subvolume (usually ID 5) is the one mounted when no 'subvol=' option is specified or when 'subvol=/' is used. The kernel prevents deletion of the currently mounted default subvolume to avoid filesystem instability. The 'btrfs subvolume set-default' command changes which subvolume is considered the default, allowing the old default to be unmounted and deleted. This is a common scenario when using Btrfs snapshots for rollback in production environments.

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 LPIC-1 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.

Related LPIC-1 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

GNU and Unix Commands practice questions

Practise LPIC-1 questions linked to GNU and Unix Commands.

Devices, Filesystems and FHS practice questions

Practise LPIC-1 questions linked to Devices, Filesystems and FHS.

Linux Installation and Package Management practice questions

Practise LPIC-1 questions linked to Linux Installation and Package Management.

Essential System Services and Networking practice questions

Practise LPIC-1 questions linked to Essential System Services and Networking.

Administrative Tasks practice questions

Practise LPIC-1 questions linked to Administrative Tasks.

Shells, Scripting and Data Management practice questions

Practise LPIC-1 questions linked to Shells, Scripting and Data Management.

System Architecture practice questions

Practise LPIC-1 questions linked to System Architecture.

LPIC-1 fundamentals practice questions

Practise LPIC-1 questions linked to LPIC-1 fundamentals.

LPIC-1 scenario practice questions

Practise LPIC-1 questions linked to LPIC-1 scenario.

LPIC-1 troubleshooting practice questions

Practise LPIC-1 questions linked to LPIC-1 troubleshooting.

Practice this exam

Start a free LPIC-1 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

LPIC-1 practice-test guide →Study guide →Browse all practice tests

FAQ

Questions learners often ask

What does this LPIC-1 question test?

Devices, Filesystems and FHS — This question tests Devices, Filesystems and FHS — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The subvolume is the default subvolume (ID 5) and cannot be deleted while it is the mounted root; use 'btrfs subvolume set-default <new_subvol_id> /srv/app', then unmount and remount with the new default subvolume, then delete the old subvolume. — The error occurs because the subvolume at /srv/app is the default subvolume (ID 5) of the Btrfs filesystem, and it is currently mounted as the root of the filesystem via the 'subvol=/' mount option. A default subvolume cannot be deleted while it is mounted because the kernel treats it as the top-level volume. To delete it, you must first set a different subvolume as the default using 'btrfs subvolume set-default', then unmount and remount the filesystem with the new default, after which the original subvolume can be safely deleted.

What should I do if I get this LPIC-1 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.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Last reviewed: Jun 25, 2026

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

This LPIC-1 practice question is part of Courseiva's free LPI 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 LPIC-1 exam.