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Difficulty: EasyConfigure Virtual Machine Scale Sets (VMSS)

An administrator is configuring autoscale rules for an Azure Virtual Machine Scale Set (VMSS) named `vmss-backend`. The scale-out rule is configured to add `22` instances when the average CPU utilization exceeds `75%75\%`. The administrator wants to configure a scale-in rule that decreases the instance count while preventing instance thrashing (continuous launching and terminating of VMs).

Which CPU utilization threshold should the administrator set for the scale-in rule?

  1. A scale-in threshold of `25%25\%` average CPU utilizationAnswer
  2. B
    A scale-in threshold of `70%70\%` average CPU utilization
  3. C
    A scale-in threshold of `75%75\%` average CPU utilization
  4. D
    A scale-in threshold of `85%85\%` average CPU utilization

Answer

A scale-in threshold of `25%25\%` average CPU utilization
The correct answer is the option specifying a scale-in threshold of `25%25\%` average CPU utilization. In autoscale configurations, there must be a significant gap between the scale-out threshold and the scale-in threshold. When an instance is removed (scale-in), the workload is distributed among fewer virtual machines, which increases the average CPU utilization on those remaining. If the scale-in threshold is too close to the scale-out threshold, this sudden increase in average CPU usage will immediately trigger a scale-out event, causing continuous scaling cycles (thrashing). A low threshold like `25%25\%` prevents this.

Step-by-Step Solution

1
Analyze the scale-out configuration.
The scale-out rule is triggered when average CPU utilization exceeds `75%75\%`.
This establishes the upper bound for scaling up capacity.
2
Evaluate the risk of instance thrashing.
If the scale-in threshold is too close to the scale-out threshold, removing an instance increases the average CPU load on the remaining instances, immediately pushing it back above the scale-out threshold.
Thrashing occurs when the difference between scale-out and scale-in thresholds is not large enough to absorb the change in average resource consumption per VM after scaling.
3
Select a threshold that provides an adequate buffer.
A threshold of `25%25\%` provides a wide buffer, ensuring that the load must drop substantially before instances are removed, keeping the scale set stable.
This guarantees that scaling in only happens during sustained periods of low activity.

Key Concept

Avoiding instance thrashing in Virtual Machine Scale Sets by setting a sufficient margin between scale-out and scale-in thresholds.
Estimated Time:45s
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Topics

Configure Virtual Machine Scale Sets (VMSS)

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