LPIC-1 System Architecture Practice Question
A production server running CentOS 7 has multiple SCSI disks (sda, sdb, sdc) configured in a RAID5 array managed by mdadm (md0). The root filesystem is on md0. After a power failure, the server boots but drops into a rescue shell with the error: 'md: md0: cannot run array. Not enough devices online.' The admin checks with 'cat /proc/mdstat' and sees that only sda and sdb are spares (marked as (S)), sdc is missing. Which sequence of commands should the admin use to attempt recovery and bring the system to a functional state? Options: A) First run 'mdadm --manage /dev/md0 --add /dev/sdc', then 'mdadm --run /dev/md0'. B) Boot from a live CD, then run 'mdadm --assemble --scan' to reassemble. C) Run 'mdadm --stop /dev/md0', then 'mdadm --assemble /dev/md0 /dev/sda /dev/sdb /dev/sdc'. D) Run 'mdadm --run /dev/md0' to force start the array in degraded mode, then add sdc with 'mdadm --add' after system is up.
⚠ Common exam trap
The trap here is that candidates mistakenly think they must stop and reassemble the array or use a live CD, when in fact the correct recovery is to force the array to run degraded with '--run' and then add the missing disk.
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 --run /dev/md0' to force start the array in degraded mode, then add sdc with 'mdadm --add' after system is up
Since the RAID5 array has only two of three disks available, it can still operate in degraded mode. The command 'mdadm --run /dev/md0' forces the array to start despite the missing device, allowing the system to boot from the root filesystem on md0. Once the system is operational, the admin can add the missing disk with 'mdadm --add /dev/md0 /dev/sdc' to initiate recovery and rebuild the array. Option A is incorrect because you cannot add a disk to an inactive array; you must first force the array to run. Option B is unnecessary; the system can be recovered without a live CD. Option C fails because stopping the array is impossible while the root filesystem is mounted, and reassembling without the missing disk will not succeed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
First run 'mdadm --manage /dev/md0 --add /dev/sdc', then 'mdadm --run /dev/md0'
Why it's wrong here
This sequence is invalid. The array is inactive, so --add cannot be used. You must first force the array to start in degraded mode with --run, then add the missing disk.
- ✗
Boot from a live CD, then run 'mdadm --assemble --scan' to reassemble
Why it's wrong here
Booting from a live CD is not required. The system can boot in rescue mode and the array can be forced degraded without a live CD.
- ✗
Run 'mdadm --stop /dev/md0', then 'mdadm --assemble /dev/md0 /dev/sda /dev/sdb /dev/sdc'
Why it's wrong here
Stopping the array is not possible because the root filesystem is mounted on it. Also, reassembling without the missing disk will not work.
- ✓
Run 'mdadm --run /dev/md0' to force start the array in degraded mode, then add sdc with 'mdadm --add' after system is up
Why this is correct
Correct. Use --run to start the array in degraded mode despite the missing disk, then add the disk to begin recovery. This allows the system to boot.
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
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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.