- A
mdadm /dev/md0 --add /dev/sdc1
Why wrong: Argument order is incorrect.
- B
mdadm --add /dev/md0 /dev/sdc1
Correct syntax.
- C
mdadm --manage /dev/md0 --add /dev/sdc1
Why wrong: Valid but longer; --add implies manage.
- D
mdadm --re-add /dev/md0 /dev/sdc1
Why wrong: Used for re-adding a disk that was removed.
Question 343 of 513
LFCS RAID 5 Practice Question
This LFCS practice question tests your understanding of operation of running systems. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: rAID 5. 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.
A system has a RAID 5 array that is degraded. One of the three disks failed and was replaced. The administrator runs 'cat /proc/mdstat' and sees that the array is still degraded. Which command should be used to add the new disk (/dev/sdc1) to the array?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"which command"Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
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
✓
mdadm --add /dev/md0 /dev/sdc1
Option B is correct because `mdadm --add /dev/md0 /dev/sdc1` is the standard syntax to add a new disk to an active RAID array. After replacing a failed disk, the array remains degraded until the new disk is added and the rebuild process starts. Option C is incorrect because including `--manage` with `--add` is not valid syntax; `--manage` is an action subcommand that should not be combined with `--add` in this way. The correct form is either `mdadm --add` or `mdadm --manage /dev/md0 --add`, but the latter is not standard and may be rejected by the command.
Key principle: RAID 5
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
mdadm /dev/md0 --add /dev/sdc1
Why it's wrong here
Argument order is incorrect.
- ✓
mdadm --add /dev/md0 /dev/sdc1
Why this is correct
Correct syntax.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
RAID 5
- ✗
mdadm --manage /dev/md0 --add /dev/sdc1
Why it's wrong here
Valid but longer; --add implies manage.
- ✗
mdadm --re-add /dev/md0 /dev/sdc1
Why it's wrong here
Used for re-adding a disk that was removed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates may confuse `--add` with `--re-add`, or think that `--manage` must be specified, but `--add` alone is the correct way to add a new disk.
Detailed technical explanation
How to think about this question
When a new disk is added to a degraded RAID 5 array, `mdadm` automatically initiates a recovery process, reading parity and data from the remaining two disks to rebuild the missing data onto the new device. The `--add` command treats the device as a spare; if the array is degraded, the spare is immediately used for recovery. In contrast, `--re-add` checks the disk's superblock for a matching array UUID and event count, which a brand-new disk lacks, so it would fail with an error.
KKey Concepts to Remember
- RAID 5
- mdadm
- Degraded array
- mdadm --add
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
RAID 5
Real-world example
How this comes up in practice
A practitioner preparing for the LFCS 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. RAID 5 Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
What to study next
Got this wrong? Here's your next step.
Review rAID 5, then practise related LFCS questions on the same topic to reinforce the concept.
- →
Operation of Running Systems — study guide chapter
Learn the concepts, then practise the questions
- →
Operation of Running Systems practice questions
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FAQ
Questions learners often ask
What does this LFCS question test?
Operation of Running Systems — This question tests Operation of Running Systems — RAID 5.
What is the correct answer to this question?
The correct answer is: mdadm --add /dev/md0 /dev/sdc1 — Option B is correct because `mdadm --add /dev/md0 /dev/sdc1` is the standard syntax to add a new disk to an active RAID array. After replacing a failed disk, the array remains degraded until the new disk is added and the rebuild process starts. Option C is incorrect because including `--manage` with `--add` is not valid syntax; `--manage` is an action subcommand that should not be combined with `--add` in this way. The correct form is either `mdadm --add` or `mdadm --manage /dev/md0 --add`, but the latter is not standard and may be rejected by the command.
What should I do if I get this LFCS question wrong?
Review rAID 5, then practise related LFCS questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
RAID 5
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Last reviewed: Jun 25, 2026
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