LFCS Storage Management Practice Question
A Linux server at a hosting provider uses a software RAID 5 array with three 2 TB disks (sda, sdb, sdc) configured as /dev/md0, hosting a large ext4 filesystem. The server experiences a performance degradation and I/O errors. The administrator checks /proc/mdstat and sees that /dev/sda is marked as failed. The remaining two disks are still active. The administrator has a spare disk /dev/sdd of the same size. The filesystem is sparse and can tolerate downtime. What is the most appropriate course of action to restore the array to a fully functional state with redundancy?
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
✓
Run 'mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda', then 'mdadm --manage /dev/md0 --add /dev/sdd'.
The proper procedure to replace a failed disk in a RAID 5 array is to first mark the failed disk as failed using '--fail', then remove it with '--remove', and finally add the new disk with '--add', which triggers an automatic rebuild. Option A is incorrect because recreating the array from scratch would destroy all existing data, requiring a backup restore, which is unnecessary when only one disk has failed. Option C is incorrect because simply adding the new disk without first removing the failed one will not allow the array to use the new disk; the array still recognizes the failed disk as part of the array and will not rebuild. Option D is incorrect because LVM is not a RAID solution and cannot directly manage or repair an existing MD RAID array; it would require recreating the storage stack and does not leverage the existing RAID metadata.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Recreate the RAID 5 array from scratch using all three healthy disks (sdb, sdc, sdd) and restore data from backup.
Why it's wrong here
Unnecessarily destructive; rebuilding can preserve data.
- ✓
Run 'mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda', then 'mdadm --manage /dev/md0 --add /dev/sdd'.
Why this is correct
The standard procedure: fail and remove the failed disk, then add the spare, triggering a rebuild.
- ✗
Run 'mdadm --manage /dev/md0 --add /dev/sdd' to directly add the new disk and let the array rebuild automatically.
Why it's wrong here
The failed disk must be removed first; adding without removal may cause conflicts.
- ✗
Use LVM to mirror the two healthy disks and ignore the failed one, ensuring data redundancy.
Why it's wrong here
LVM does not provide RAID 5 functionality and would require reconfiguration, losing the benefit of RAID 5.
Quick reference
RAID Level Comparison
| RAID Level | Min Disks | Fault Tolerance | Read | Write | Usable Capacity |
|---|---|---|---|---|---|
| RAID 0 | 2 | None | Excellent | Excellent | 100% |
| RAID 1 | 2 | 1 disk | Good | Moderate | 50% |
| RAID 5 | 3 | 1 disk | Good | Moderate | 67–94% |
| RAID 6 | 4 | 2 disks | Good | Lower | 50–88% |
| RAID 10 | 4 | 1 disk per mirror | Excellent | Good | 50% |
RAID is not a backup strategy — it protects against disk failure but not against accidental deletion, ransomware, or site-level events.
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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.