Azure VMSS Autoscale

Question

Folks,
I'm trying to determine how Autoscale works:

• If the initial instance count is 5 virtual machines, the minimum is 2 virtual machines and the maximum is 10 virtual machines. When I first deployed my VMSS, I will have 5 virtual machines running. Right?

If I have a rule that says when the CPU reaches 80% it will add an additional virtual machine.

• How do the increase and decrease rule work? e.g. does the CPU of those initially deployed virtual machines need to operate at or above 80% CPU capacity for 5 minutes before a new instance is started or operate at below 25% CPU capacity before an existing instance is removed?

Lisa.

Answer 1

Hi @Lisa Lownds

1. You are right. If your initial instance count is % VMs, when you first deploy VMSS, you will have 5 VMs running.
2. Let's say you created a rule that increases the number of VM instances in a scale set when the average CPU load is greater than 80% over a 5-minute period. When the rule triggers, the number of VM instances is increased by 20%, if that's what you selected in the 'Action' section of the autoscale rules. In scale sets with a small number of VM instances, you could set the Operation to Increase count by and then specify 1 or 2 for the Instance count. In scale sets with a large number of VM instances, an increase of 10% or 20% VM instances may be more appropriate.

I would suggest reading on this example in Azure documentation: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-autoscale-portal#create-a-rule-to-automatically-scale-out

Please 'Accept as answer' if this helped you so that it can help others in the community.

Answer 2

Thank you Karishma.