LFCS Storage Management Practice Question
An administrator needs to configure software RAID 5 on three disks /dev/sda, /dev/sdb, /dev/sdc with a spare disk /dev/sdd. Which command correctly creates the RAID array?
⚠ Common exam trap
Many candidates confuse `--raid-devices` with the total number of disks provided, leading them to set `--raid-devices=4` (option B) when they intend to include a spare, or they forget to specify the spare at all (option C).
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 --create /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 /dev/sda /dev/sdb /dev/sdc /dev/sdd
It uses the `--spare-devices=1` flag to designate `/dev/sdd` as a hot spare, while `--raid-devices=3` specifies that only three disks form the active RAID 5 array. The spare disk is listed after the active disks, which is the correct syntax for `mdadm --create`.
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 --create /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 /dev/sda /dev/sdb /dev/sdc /dev/sdd
Why this is correct
The `--level=5` flag sets RAID 5 parity striping across three active members, while `--raid-devices=3` fixes the array's active disk count and `--spare-devices=1` designates /dev/sdd as a hot spare. Listing all four devices in order satisfies the stem's requirement for a three-disk RAID 5 array with one standby disk.
- ✗
mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd
Why it's wrong here
Setting --raid-devices=4 makes all four disks active members, producing RAID 5 with four devices and no spare, so no disk remains for automatic rebuild. It is tempting because all four devices appear, but the scenario requires three active members plus one spare, declared via --spare-devices=1.
- ✗
mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sda /dev/sdb /dev/sdc
Why it's wrong here
This creates a three-disk RAID 5 array but omits /dev/sdd entirely, leaving no spare to rebuild onto after a disk failure. It is tempting because --raid-devices=3 correctly reflects the active member count, yet the requirement for a designated spare means the device list must also include /dev/sdd.
- ✗
mdadm --create /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 /dev/sda /dev/sdb /dev/sdc --spare /dev/sdd
Why it's wrong here
mdadm has no --spare flag; spares are declared with --spare-devices and then listed among the member devices, so this command fails with an unrecognised option. It is tempting because the intent matches the scenario, but the correct syntax appends /dev/sdd to the device list after the three RAID members.
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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JA
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.