EX200 Configure local storage Practice Question
A server has a software RAID 5 array /dev/md0. One of its disks fails. The administrator wants to replace it without rebooting. Which command should be used to mark the disk as failed?
⚠ Common exam trap
A common mix-up: candidates confuse the valid `--fail` option with the non-existent `--fault` or `--set-faulty` options, or they incorrectly think the sysfs method is the standard command-line approach expected in the EX200 exam.
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 --fail /dev/md0 /dev/sdb
The correct command to mark a disk as failed in a software RAID array without rebooting is `mdadm --fail /dev/md0 /dev/sdb`. This command tells the md driver to mark the specified disk as faulty, which triggers the RAID 5 array to degrade and allows the failed disk to be removed and replaced while the system remains online.
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 --fault /dev/md0 /dev/sdb
Why it's wrong here
The mdadm command does not accept a --fault option at all; mdadm's failure-marking operation is exposed as --fail (or the shorter -f) under the --manage action. Passing an unrecognized option like --fault will make mdadm print a usage error and exit non-zero, leaving the drive still active in the array. If you intend to simulate a drive failure, you must use the documented --fail flag rather than inventing a synonym.
- ✗
echo faulty > /sys/block/md0/md/dev-sdb/state
Why it's wrong here
Writing 'faulty' to /sys/block/md0/md/dev-sdb/state is a kernel-level sysfs manipulation that can technically mark the device as faulty, but it bypasses mdadm's management interface and is not the standard, supported administrative practice. The sysfs state file uses values like 'faulty' or 'failed', but the exact path and semantics are kernel-version-dependent and easy to get wrong, especially if the array is managed by mdadm. The canonical, portable method is to use mdadm --manage --fail, which performs the same operation through the proper tooling.
- ✗
mdadm --set-faulty /dev/md0 /dev/sdb
Why it's wrong here
The option --set-faulty does not exist in mdadm's option set; mdadm's --fail (or --set-faulty in some older documentation? Actually, no, --set-faulty is not valid; the correct flag is --fail). This command would be rejected by mdadm as an invalid option, producing an error and making no change to the array. Unlike --fail, which is explicitly designed to mark a device as faulty, --set-faulty is a misleading construct that no real mdadm version supports.
- ✓
mdadm --fail /dev/md0 /dev/sdb
Why this is correct
This is the correct command: mdadm --manage --fail /dev/md0 /dev/sdb (the --manage action is implicit when using --fail) marks /dev/sdb as faulty in the RAID 5 array /dev/md0. Once marked, mdadm removes the device from the active array, and the array continues operating in a degraded state because RAID 5 tolerates a single disk failure. You can then remove the failed disk (mdadm --remove) and replace it (mdadm --add) to rebuild redundancy, which is the proper workflow for handling a failing 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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