The correct action is to replace /dev/sdc with a new disk and restore the stripe, because an LVM stripe performance bottleneck is always dictated by the slowest disk in the set. When iostat shows high average wait times on only one physical volume, that device becomes a drag on every write operation, as LVM stripes data across all PVs in parallel—the entire stripe must wait for the slowest member to complete. On the LPIC-2 exam, this scenario tests your understanding that striping does not tolerate unbalanced I/O; a common trap is to assume adding more PVs or adjusting the stripe size will fix a hardware-level latency issue. Remember the "weakest link" rule: in a striped logical volume, performance equals the speed of the slowest PV. A useful mnemonic is "Stripe speed sinks with the slowest link"—always check each PV’s iostat before tuning stripe parameters.
LPIC-2 Practice Question: Block Devices, Filesystems and Advanced Storage
This LPIC-2 practice question tests your understanding of block devices, filesystems and advanced storage. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 Linux administrator is managing a database server running on CentOS 7 that uses ext4 filesystems on LVM. The server has three physical volumes: /dev/sda (200GB), /dev/sdb (200GB), and /dev/sdc (200GB) all in volume group 'vg_db'. The logical volume 'lv_data' (400GB) is used for database files. Recently, the DBA reports that database writes are slower than expected. Iostat shows high average wait times (>100ms) on /dev/sdc but normal on /dev/sda and /dev/sdb. The LVM stripes data across all three PVs with a stripe size of 64KB. Which action should the administrator take to improve performance?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Replace /dev/sdc with a new disk and restore the mirror or restripe.
The high average wait time on /dev/sdc indicates that this specific physical volume is a performance bottleneck, likely due to hardware failure or degradation. Since the logical volume 'lv_data' stripes data across all three PVs, the overall write performance is limited by the slowest device in the stripe set. Replacing /dev/sdc with a new disk and restoring the stripe (e.g., using pvmove to relocate extents and then replacing the device) eliminates the bottleneck and restores balanced I/O across all PVs.
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.
✓
Replace /dev/sdc with a new disk and restore the mirror or restripe.
Why this is correct
The slow disk is the bottleneck; replacing it resolves the issue.
Related concept
Read the scenario before looking for a memorised answer.
✗
Convert lv_data to linear mapping on /dev/sda and /dev/sdb only.
Why it's wrong here
This loses the benefit of striping and may not improve performance.
✗
Increase the stripe size to 256KB to reduce I/O overhead.
Why it's wrong here
This may not address the slow disk; the issue is at the device level.
✗
Remove /dev/sdc from the volume group and reduce the stripe count to 2.
Why it's wrong here
Reducing stripes reduces parallelism, may hurt performance further.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think increasing stripe size or reducing stripe count will improve performance, but they overlook that a single failing or slow physical volume in a striped logical volume creates a synchronous bottleneck that cannot be mitigated by tuning stripe parameters alone.
Detailed technical explanation
How to think about this question
In LVM striping, each write is split across all PVs in the stripe set; if one PV has high latency, every write must wait for that slow device to complete its portion, causing the overall I/O to be as slow as the slowest component. The iostat average wait time (await) includes both service time and queue time; values >100ms often indicate hardware issues such as a failing disk, bad sectors, or a congested SAS/SATA bus. Using pvmove to relocate extents off /dev/sdc before replacement ensures no data loss and maintains stripe alignment.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Block Devices, Filesystems and Advanced Storage — This question tests Block Devices, Filesystems and Advanced Storage — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Replace /dev/sdc with a new disk and restore the mirror or restripe. — The high average wait time on /dev/sdc indicates that this specific physical volume is a performance bottleneck, likely due to hardware failure or degradation. Since the logical volume 'lv_data' stripes data across all three PVs, the overall write performance is limited by the slowest device in the stripe set. Replacing /dev/sdc with a new disk and restoring the stripe (e.g., using pvmove to relocate extents and then replacing the device) eliminates the bottleneck and restores balanced I/O across all PVs.
What should I do if I get this LPIC-2 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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