Question 54 of 511

vSphere Performance and Scaling →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the vSAN disk group has a single SSD cache leading to high latency during heavy writes, and the recommended solution is to convert to all-flash vSAN. This is correct because the high %DAVG latency spikes to 100 ms during write bursts indicate that the single SSD cache in the hybrid disk group is being overwhelmed, acting as a write buffer that exhausts its performance under sustained SQL Server database writes. On the VMware Certified Professional Data Center Virtualization VCP-DCV exam, this scenario tests your understanding of vSAN architecture—specifically how hybrid configurations bottleneck on the SSD cache tier during heavy write workloads, while all-flash vSAN eliminates the HDD tier entirely. A common trap is assuming the cache is full, but the issue is write buffer exhaustion, not capacity. Memory tip: "Cache capacity isn't the problem; cache endurance under write bursts is—flash the HDDs away."

VCP-DCV vSphere Performance and Scaling Practice Question

This VCP-DCV practice question tests your understanding of vsphere performance and scaling. 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 managed hosting provider uses vSphere 7 with vSAN to run customer VMs. One customer's VM is a SQL Server database with 8 vCPUs and 128 GB RAM. The administrator notices that the VM's performance during peak hours is poor, with high disk latency and occasional disconnects. The cluster has 4 hosts, each with 10 cores (HT enabled) and 256 GB RAM. vSAN is configured with a hybrid disk group (SSD cache, HDD capacity) per host. The VM's storage policy is set to 'Performance' with RAID-1 mirroring (2 copies). The administrator runs esxtop and sees high %DAVG (device average latency) for the VM's vmdk. The observed latency averages 30 ms, but spikes to 100 ms. The host where the VM is running has relatively low CPU and memory usage, and the vSAN cache is not full. Which of the following is the most likely root cause and recommended solution?

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.

Question 1hardmultiple choice

Full question →

A
Cause: vSAN disk group has a single SSD cache leading to high latency during heavy writes. Solution: Convert to all-flash vSAN.
Hybrid vSAN uses SSDs as cache and HDDs as capacity; write bursts can exceed the cache flush rate to HDDs, causing latency spikes. All-flash eliminates the HDD bottleneck.
B
Cause: vSAN cache is unable to handle the write burst. Solution: Add more cache capacity by using larger SSDs.
Why wrong: The cache is not full; the issue is write buffer exhaustion under burst, not capacity. Larger SSDs may not help if the burst exceeds cache write buffer.
C
Cause: The VM's memory ballooning is causing excessive swapping to vSAN, which is slow. Solution: Increase the VM's memory reservation.
Why wrong: Memory ballooning would cause high latency in swap I/O, but %DAVG is on the vmdk, not swap. Also, host memory is not overcommitted.
D
Cause: vSAN is encountering disk contention due to multiple VMs sharing the same disk. Solution: Deploy a dedicated vSAN datastore for the customer.
Why wrong: vSAN distributes objects across multiple disks and hosts, so sharing is inherent. A dedicated datastore does not change the architecture.

Full breakdown with real-world context →

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

✓

Cause: vSAN disk group has a single SSD cache leading to high latency during heavy writes. Solution: Convert to all-flash vSAN.

Option C is correct. The high spike latency suggests that the SSD cache is being overwhelmed during write bursts. All-flash vSAN eliminates the HDD tier bottleneck. Option A is wrong; the cache is not full, so adding capacity won't help write bursts; the issue is write buffer exhaustion. Option B is wrong; memory ballooning would cause swapping but not high device latency. Option D is wrong; contention is not from multiple VMs sharing the disk group, as vSAN distributes objects.

Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
Cause: vSAN disk group has a single SSD cache leading to high latency during heavy writes. Solution: Convert to all-flash vSAN.
Why this is correct
Hybrid vSAN uses SSDs as cache and HDDs as capacity; write bursts can exceed the cache flush rate to HDDs, causing latency spikes. All-flash eliminates the HDD bottleneck.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
Cause: vSAN cache is unable to handle the write burst. Solution: Add more cache capacity by using larger SSDs.
Why it's wrong here
The cache is not full; the issue is write buffer exhaustion under burst, not capacity. Larger SSDs may not help if the burst exceeds cache write buffer.
✗
Cause: The VM's memory ballooning is causing excessive swapping to vSAN, which is slow. Solution: Increase the VM's memory reservation.
Why it's wrong here
Memory ballooning would cause high latency in swap I/O, but %DAVG is on the vmdk, not swap. Also, host memory is not overcommitted.
✗
Cause: vSAN is encountering disk contention due to multiple VMs sharing the same disk. Solution: Deploy a dedicated vSAN datastore for the customer.
Why it's wrong here
vSAN distributes objects across multiple disks and hosts, so sharing is inherent. A dedicated datastore does not change the architecture.

Common exam traps

Common exam trap: NAT rules depend on direction and matching traffic

NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.

Detailed technical explanation

How to think about this question

NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.

KKey Concepts to Remember

Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.

TExam Day Tips

Identify inside and outside interfaces first.
Check whether the scenario needs static NAT, dynamic NAT or PAT.
Do not confuse NAT matching ACLs with normal packet-filtering intent.

Key takeaway

NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.

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.

Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related VCP-DCV NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this VCP-DCV question test?

vSphere Performance and Scaling — This question tests vSphere Performance and Scaling — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: Cause: vSAN disk group has a single SSD cache leading to high latency during heavy writes. Solution: Convert to all-flash vSAN. — Option C is correct. The high spike latency suggests that the SSD cache is being overwhelmed during write bursts. All-flash vSAN eliminates the HDD tier bottleneck. Option A is wrong; the cache is not full, so adding capacity won't help write bursts; the issue is write buffer exhaustion. Option B is wrong; memory ballooning would cause swapping but not high device latency. Option D is wrong; contention is not from multiple VMs sharing the disk group, as vSAN distributes objects.

What should I do if I get this VCP-DCV question wrong?

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?

Static NAT maps one inside address to one outside address.

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Last reviewed: Jun 24, 2026

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