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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-app`. The scale-out rule is triggered when the average CPU usage exceeds 80%80\%. To prevent scale thrashing (frequent, alternating scaling up and down of instances), which of the following autoscale parameters must be configured? (Select TWO.)

  1. A scale-in rule with a CPU threshold set sufficiently below the scale-out threshold (e.g., at 60%60\% or lower)Answer
  2. A cooldown period to specify the amount of time to wait before subsequent scaling actions occurAnswer
  3. C
    A manual upgrade policy to prevent automatic instance updates during scaling events
  4. D
    A default metric resource group to collect performance logs from the instances

Answer

To prevent scale thrashing in an Azure Virtual Machine Scale Set, you must configure a scale-in rule with a threshold set sufficiently below the scale-out threshold, and configure a cooldown period.
To prevent VMSS autoscale thrashing, administrators must configure two main parameters: a scale-in rule with a threshold set significantly lower than the scale-out threshold, and a cooldown period. The threshold buffer ensures that removing an instance does not immediately trigger a scale-out event. The cooldown period pauses further scaling evaluations for a specified time to let new instances load and handle traffic.

Step-by-Step Solution

1
Analyze how scale thrashing occurs in autoscale rules.
Thrashing occurs when the scale-in and scale-out thresholds are too close. For instance, if scale-out is at 80%80\% and scale-in is at 75%75\%, removing a VM can immediately push average CPU back above 80%80\%, causing an infinite loop of scaling.
Identifying the mathematical cause of thrashing determines the required parameter adjustments.
2
Verify the role of the scale-in threshold buffer.
Configuring the scale-in threshold significantly lower (e.g., 60%60\%) provides a buffer so that the capacity decrease does not immediately trigger another scale-out.
Ensuring a wide gap between scale-out and scale-in thresholds resolves cyclical capacity issues.
3
Determine the impact of the cooldown period.
The cooldown period enforces a delay (e.g., 5 to 10 minutes) before another autoscale action can occur, letting metrics stabilize as new instances boot up.
Time-based throttling is critical to allow new VMs to start serving traffic before Azure Monitor re-evaluates the rules.

Key Concept

Preventing instance thrashing in Virtual Machine Scale Sets by configuring proper metric thresholds and cooldown durations.
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Topics

Configure Virtual Machine Scale Sets (VMSS)

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