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?
⚠ Common exam trap
LFCS often tests the misconception that you can simply add a new disk to a degraded RAID array without first removing the failed disk, when in fact the failed disk must be marked failed and removed before adding the replacement.
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 correct procedure is to mark the failed disk as failed (if not already), remove it from the array, then add the spare disk /dev/sdd. This allows mdadm to rebuild the RAID 5 array onto the new disk, restoring redundancy. The commands 'mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda' followed by 'mdadm --manage /dev/md0 --add /dev/sdd' accomplish this safely.
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
Recreating the array destroys the existing ext4 filesystem and forces a full restore from backup, which the stem does not require since two members still hold complete parity data. Rebuilding from scratch is the right choice only when too many members have failed for the array to assemble.
- ✓
Run 'mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda', then 'mdadm --manage /dev/md0 --add /dev/sdd'.
Why this is correct
Marking sda failed and removing it, then adding sdd, lets mdadm rebuild redundancy onto the spare while the degraded array stays online. This restores RAID 5 fault tolerance without recreating the ext4 filesystem, matching the requirement to regain redundancy with minimal disruption.
- ✗
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
Adding /dev/sdd with --add leaves the array degraded: mdadm marks the new disk as a spare until the failed member is explicitly removed with --fail and --remove, so redundancy is never restored. It is tempting because --add is the correct step for expanding an already-healthy array with an extra spare.
- ✗
Use LVM to mirror the two healthy disks and ignore the failed one, ensuring data redundancy.
Why it's wrong here
LVM mirroring operates on logical volumes, not on the mdadm member devices, so it cannot repair /dev/md0 or reinstate redundancy across sdb, sdc and sdd. LVM mirrors would be the correct mechanism when designing redundancy at the volume-group layer instead of software RAID.
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.
Go deeper
Related to this question
About these practice questions
Courseiva writes every LFCS question from scratch — 406 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Linux Foundation exam blueprint
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.