LPIC-1 Devices, Filesystems and FHS Practice Question
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?
⚠ Common exam trap
A common mix-up: 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.
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.