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Zorluk: OrtaConfigure Virtual Machine Scale Sets (VMSS)

An administrator is configuring autoscaling for an Azure Virtual Machine Scale Set named `vmss-payments` that uses Flexible orchestration. The scale set currently has 55 instances and uses a Rolling upgrade policy. You configure a scale-out rule to increase the instance count by 22 when the average CPU percentage is greater than 80%80\% for 1010 minutes. You need to configure a scale-in rule that decreases the instance count by 11 when the average CPU percentage is less than a threshold value for 1010 minutes. To prevent instance thrashing, what is the maximum value that you should set for the scale-in CPU threshold?

  1. 40%40\%Cevap
  2. B
    60%60\%
  3. C
    70%70\%
  4. D
    75%75\%

Cevap

The correct threshold to configure is 40%40\%.
To prevent thrashing, the scale-in threshold must be lower than the expected CPU utilization after a scale-out occurs. With 55 instances running at just over 80%80\%, the total workload is 400%400\% to 405%405\%. When 22 instances are added, the load is distributed across 77 instances, reducing the average CPU usage to approximately 57%57\% to 58%58\%. Setting the scale-in threshold to 40%40\% ensures that the scale-in rule does not trigger immediately after scaling out.

Adım Adım Çözüm

1
Calculate the total CPU load when the scale-out is triggered.
At 55 instances with 80%80\% average load, the total load is 5×80%=400%5 \times 80\% = 400\% CPU capacity.
This determines the total workload that needs to be distributed across the new instances.
2
Calculate the new average CPU usage after scale-out.
Adding 22 instances increases the total count to 77. The new average CPU load is 400%/7≈57.1%400\% / 7 \approx 57.1\% (or 405%/7≈57.9%405\% / 7 \approx 57.9\% if just above 80%80\%, e.g., 81%81\%, when triggered).
This identifies the new operational CPU percentage immediately after the scale-out occurs.
3
Select a scale-in threshold that is lower than the new average CPU usage.
The threshold must be set below 57%57\%. Among the options, 40%40\% is the only value below this limit.
Setting the threshold to 60%60\%, 70%70\%, or 75%75\% would immediately trigger a scale-in, causing thrashing.

Anahtar Kavram

Autoscale thrashing prevention by keeping scale-in thresholds below the post-scale-out CPU utilization level.
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Konular

Configure Virtual Machine Scale Sets (VMSS)

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