EX200 Configure local storage Practice Question
A production server runs RHEL 8 with a software RAID 5 array (/dev/md0) composed of three disks: /dev/sda, /dev/sdb, /dev/sdc. The array is used to store database files. The server experiences a disk failure on /dev/sdc. The admin replaces /dev/sdc with an identical disk and wants to rebuild the array. He runs: mdadm /dev/md0 --add /dev/sdc. The command completes without error, but the array shows a degraded state after several hours. What should the admin do next?
⚠ Common exam trap
Candidates often assume the rebuild is always automatic and instantaneous, or they panic and choose destructive options like recreating the array, instead of first verifying the rebuild status with a simple diagnostic command.
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
✓
Run mdadm --detail /dev/md0 to check the status and rebuild progress.
After adding a replacement disk to a RAID 5 array, the rebuild process begins automatically but may take hours depending on disk size and I/O load. Running `mdadm --detail /dev/md0` allows the admin to check the current state, rebuild progress (e.g., percentage complete), and any errors that might have stalled the rebuild. This is the first diagnostic step to determine if the rebuild is still ongoing, has failed, or is degraded for another reason.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Run mdadm --detail /dev/md0 to check the status and rebuild progress.
Why this is correct
Running `mdadm --detail /dev/md0` is the correct first step because it reports the array state (`clean`, `degraded`, or `recovering`) and shows per-device status, including whether `/dev/sdc` is present as an active or spare device. It also displays a rebuild progress percentage and estimated time when a reshape or reconstruction is in progress, allowing you to confirm that recovery is actually underway and to spot device errors or mismatches. Without this command, you cannot distinguish a healthy rebuild from one that has stalled or failed.
- ✗
Rebuild will happen automatically; just wait longer.
Why it's wrong here
RAID 5 does not always recover on its own after a disk is replaced: automatic rebuild requires a configured spare or an explicit command to add the disk to the array, and even then I/O errors, conflicting metadata, or a non-fresh disk can prevent it from starting. Waiting passively for hours ignores the need to verify the array's state and can allow a degraded array to continue running with reduced redundancy, increasing the risk of data loss if another disk fails. Monitoring with `mdadm --detail` is necessary rather than assuming the kernel will handle everything.
- ✗
Recreate the array using mdadm --create with the same parameters.
Why it's wrong here
Using `mdadm --create /dev/md0` with the same parameters writes fresh RAID superblocks to all participating disks and treats the current data as nonexistent, thereby destroying the filesystem and all data on the logical volume. This is only appropriate for building a brand-new array, not for repairing a degraded one, and it is especially dangerous if the failed disk has been replaced and added, because the original data could have been reconstructed. The correct recovery operation is `mdadm --manage /dev/md0 --add /dev/sdc` (or `--re-add`), which preserves the array's existing data.
- ✗
Format /dev/sdc with a filesystem before adding to the array.
Why it's wrong here
Formatting `/dev/sdc` with mkfs puts a filesystem signature on the disk, but mdadm requires each member to be a raw block device without any filesystem, partitioning, or leftover RAID superblock. A formatted disk will be recognized as a non-member or rejected when added, because mdadm expects to write its own metadata at the start of the device. You should instead use `mdadm --zero-superblock /dev/sdc` (if needed) and add it cleanly to the 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
This EX200 practice question is part of Courseiva's free Red Hat 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 EX200 exam.