LPIC-1 System Architecture Practice Question
A system has a software RAID1 array (/dev/md0) with two disks: /dev/sda and /dev/sdb. Disk /dev/sda fails. Which command sequence will replace the failed disk with a new /dev/sdc without stopping the array?
⚠ Common exam trap
Test-takers frequently assume a failed disk requires array shutdown or physical replacement before software reconfiguration, but mdadm's --manage subcommand allows all steps (fail, remove, add) while the array remains active.
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 --manage /dev/md0 --fail /dev/sda --remove /dev/sda && mdadm --manage /dev/md0 --add /dev/sdc
Mdadm's --manage mode allows hot-replacement of a failed disk in a RAID1 array without stopping it. The --fail flag marks /dev/sda as faulty, --remove detaches it, and --add incorporates the new /dev/sdc, triggering an automatic rebuild of the mirror.
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 --stop /dev/md0; replace disk; mdadm --assemble /dev/md0 /dev/sdb /dev/sdc
Why it's wrong here
Stopping array is unnecessary and may cause downtime.
- ✓
mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda && mdadm --manage /dev/md0 --add /dev/sdc
Why this is correct
Correct sequence: fail, remove, then add new disk.
- ✗
Shutdown system, physically replace disk, reboot
Why it's wrong here
Reboot is not required and causes downtime.
- ✗
dd if=/dev/sdb of=/dev/sdc bs=64K
Why it's wrong here
dd clones the good disk to new, but the array may still include failed disk.
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
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