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Difficulty: MediumConfigure Virtual Machine Scale Sets (VMSS)

An administrator is configuring autoscale settings for an Azure Virtual Machine Scale Set named `vmss-orders` that uses Uniform orchestration. The scale set currently has a capacity of 44 instances. The upgrade policy is set to Manual.

The administrator configures a scale-out rule to add 22 instances when the average CPU utilization exceeds 75%75\%.

To prevent instance thrashing, where the scale set repeatedly scales out and then immediately scales in under a constant workload, what is the maximum CPU utilization threshold that should be configured for the scale-in rule (which removes 22 instances)?

  1. 45%45\%Answer
  2. B
    55%55\%
  3. C
    65%65\%
  4. D
    70%70\%

Answer

The correct scale-in threshold is 45%45\%.
Under a constant workload, if 44 instances are running at 75%75\% CPU utilization, the total workload corresponds to 300%300\% CPU capacity. When scaling out to 66 instances, this 300%300\% workload is distributed across all 66 instances, resulting in an average CPU utilization of 50%50\% per instance. To prevent the scale set from immediately scaling back in, the scale-in threshold must be configured below 50%50\%. Therefore, a threshold of 45%45\% is the only option that prevents immediate scale-in and avoids thrashing.

Step-by-Step Solution

1
Calculate the total workload capacity at the scale-out trigger point.
Total workload capacity is 4×75%=300%4 \times 75\% = 300\% CPU equivalent.
This represents the total CPU load generated by the workload running on the initial scale set size.
2
Calculate the distributed average CPU utilization immediately after scaling out.
The new average CPU utilization is 300%÷6=50%300\% \div 6 = 50\%.
Adding 22 instances increases the capacity to 66 instances, which distributes the constant workload.
3
Determine the maximum threshold for scale-in to prevent thrashing.
The scale-in threshold must be strictly less than 50%50\%.
If the threshold is 50%50\% or higher, the scale-in rule triggers immediately, reverting capacity to 44 instances and creating a loop.

Key Concept

Avoiding VMSS instance thrashing by mathematically separating scale-out and scale-in thresholds based on resource utilization redistribution.
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Topics

Configure Virtual Machine Scale Sets (VMSS)

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