EX200 Configure local storage Practice Question
A system administrator is configuring a new RHEL 9 server with two 500GB SSDs. The requirement: create a 200GB XFS filesystem for /srv/data that is resilient to disk failure. The admin decides to create a RAID 1 (mirror) using mdadm with partitions on each disk: /dev/sda1 and /dev/sdb1, each 200GB. He creates the partitions with fdisk, then runs: mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1. The array is created and synced. He then creates a physical volume, volume group, and logical volume on top of /dev/md0, formats with XFS, and mounts. Later, a disk fails. After replacing the failed disk, he recreates the partition with identical size and runs: mdadm /dev/md0 --add /dev/sdb1. The command fails with 'Device /dev/sdb1 is busy'. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often assume the 'Device busy' error means the disk is already in use by another array or process, when in fact it is a stale partition table cache that prevents the kernel from recognizing the new partition.
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
✓
The kernel still sees the old partition table; need to run partprobe to reread the partition table.
After replacing a failed disk and recreating the partition, the kernel's in-memory partition table still reflects the old state. The `mdadm --add` command fails with 'Device busy' because the kernel sees the old partition layout and may still hold references to the old partition. Running `partprobe` (or `partx -a`) forces the kernel to reread the partition table from the disk, clearing the stale state and allowing the new partition to be added to the RAID array.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The replacement disk was not initialized with an mdadm superblock before adding.
Why it's wrong here
The --add operation automatically writes a fresh mdadm superblock to the device when it is incorporated into the array, so pre-initializing the disk with a superblock is unnecessary. In fact, trying to create a superblock beforehand could leave stale metadata that confuses the array. The real failure here is that the kernel has not been told about the new partition on the replacement disk.
- ✓
The kernel still sees the old partition table; need to run partprobe to reread the partition table.
Why this is correct
After you create a new partition table entry on /dev/sdb, the kernel keeps using the old partition layout cached in memory. Running partprobe (or partx -a) forces a re-read of the partition table so that /dev/sdb1 actually appears. Without this step, mdadm will report that the partition does not exist, even though it is visible in fdisk output.
- ✗
The /dev/md0 array is still in a clean state and does not need the disk yet.
Why it's wrong here
When a disk in a RAID1 array fails, the array automatically switches to a degraded but still operational state, and /proc/mdstat will indicate the missing device. The array is not 'clean' in the sense of having no need for the replacement; rather, it is missing one of its mirrors and is waiting to be rebuilt. Until the new disk is added, the array lacks the expected redundancy and is vulnerable to a second failure.
- ✗
The /dev/sdb1 partition is already part of another md array.
Why it's wrong here
A brand-new replacement disk would not normally contain a valid mdadm superblock unless it had been reused from a previous array. You can confirm this by running `mdadm --examine /dev/sdb1`, which would show an array membership; if that were the case you could wipe it with `mdadm --zero-superblock`. However, the more plausible cause of the 'device not found' error is the kernel's stale partition table, not pre-existing RAID membership.
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.