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 md0 takes the array offline, so the replacement is not performed without stopping it. The --stop and --assemble sequence is tempting because it does rebuild a RAID1 from surviving members, but only after downtime, which the scenario explicitly forbids.
- ✓
mdadm --manage /dev/md0 --fail /dev/sda --remove /dev/sda && mdadm --manage /dev/md0 --add /dev/sdc
Why this is correct
mdadm --manage marks the failed member faulty with --fail, then detaches it using --remove, freeing the slot. Adding /dev/sdc with --add triggers the RAID1 rebuild while /dev/md0 stays assembled and serving, satisfying the no-downtime constraint.
- ✗
Shutdown system, physically replace disk, reboot
Why it's wrong here
A shutdown halts the array and all I/O, directly violating the requirement to replace the disk without stopping it. This approach is tempting because physically swapping a failed disk is the traditional repair method, but it applies to cold replacement, not live RAID1 member substitution.
- ✗
dd if=/dev/sdb of=/dev/sdc bs=64K
Why it's wrong here
dd copies raw blocks from sdb to sdc, which does not add sdc as a RAID member; md0 still references the failed sda and never rebuilds. It is tempting because dd clones disks, which works for duplicating standalone volumes, not for enrolling a device into an existing md array.
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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