This EX200 practice question tests your understanding of configure local storage. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Option B is correct because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 command writes the superblock; no prior initialization needed.
✓
The kernel still sees the old partition table; need to run partprobe to reread the partition table.
Why this is correct
After recreating partition, the kernel may still have old info; partprobe updates it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The /dev/md0 array is still in a clean state and does not need the disk yet.
Why it's wrong here
If a disk failed, the array is degraded and needs the replacement.
✗
The /dev/sdb1 partition is already part of another md array.
Why it's wrong here
The disk is new; unlikely.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Trap categories for this question
Command / output trap
The --add command writes the superblock; no prior initialization needed.
Detailed technical explanation
How to think about this question
When a partition is created or modified, the kernel caches the partition table in memory. The `mdadm --add` command attempts to open the block device, but if the kernel still holds the old partition's device node (e.g., due to an open file handle or a leftover md superblock reference), it returns EBUSY. Running `partprobe` (or `blockdev --rereadpt`) sends a BLKRRPART ioctl to the disk, forcing the kernel to invalidate the cached partition table and re-read from the disk. This is a common step after partition table changes on a disk that was previously part of an md array.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the EX200 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Configure local storage — This question tests Configure local storage — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The kernel still sees the old partition table; need to run partprobe to reread the partition table. — Option B is correct because 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.
What should I do if I get this EX200 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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