VMSS scale set is duplicating search processes once scaled out

Question

VMSS scale set is duplicating search processes once scaled out

Olufemi Olaoye 20

VMSS scale set is duplicating search processes that is being run by the application we deployed. For context, our application does social media platform searches and it was working fine when we first set up the VMSS. The main purpose for setting up the VMSS is to release burden on the CPU usage on a single VM. But this week, the searches are being duplicated once the VMSS scales out, costing us unnecessary tokens.

In essence, the additional instances created by the scale sets are duplicating the processes that had already been done by the Parent Instance (VM).

Please advise why VMSS is behaving this way.

Dharani Reguri 430 Reputation points Microsoft External Staff

2025-04-15T17:03:24.7333333+00:00

Hi Olufemi Olaoye,

I understand that when VMSS scales out an instance, the new instance is duplicating the process done by the Parent instance. This is happening because with scale sets, all VM instances are created from the same base OS image and configuration and runs the startup scripts. when VMSS spins up the instance it is not aware of the task that is already done.

I would suggest using queue mechanism and let VM know that particular search operation was already done.

Please refer the document about VMSS.

Please let me know if you have further questions.

Thank you.
Olufemi Olaoye 20 Reputation points

2025-04-16T13:05:12.1833333+00:00

Thank you Dharani for the help and prompt responses. The VMSS scales out horizontally and the task/process is not shared between the scale sets.

I wonder if Microsoft can consider vertical scaling i.e. increasing CPU and/or disk size automatically when the workloads require such and decreasing once the workload reduces.

Another option that could help is if the horizontal scaling can be synchronized such that tasks being done on a Parent VMSS is not repeated in the spinned-up instances.

All said, thank you once again Dharani for your professionalism.

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Dharani Reguri 430 Reputation points Microsoft External Staff

2025-04-15T17:03:24.7333333+00:00

Hi Olufemi Olaoye,

I understand that when VMSS scales out an instance, the new instance is duplicating the process done by the Parent instance. This is happening because with scale sets, all VM instances are created from the same base OS image and configuration and runs the startup scripts. when VMSS spins up the instance it is not aware of the task that is already done.

I would suggest using queue mechanism and let VM know that particular search operation was already done.

Please refer the document about VMSS.

Please let me know if you have further questions.

Thank you.
Olufemi Olaoye 20 Reputation points

2025-04-16T13:05:12.1833333+00:00

Thank you Dharani for the help and prompt responses. The VMSS scales out horizontally and the task/process is not shared between the scale sets.

I wonder if Microsoft can consider vertical scaling i.e. increasing CPU and/or disk size automatically when the workloads require such and decreasing once the workload reduces.

Another option that could help is if the horizontal scaling can be synchronized such that tasks being done on a Parent VMSS is not repeated in the spinned-up instances.

All said, thank you once again Dharani for your professionalism.

Answer 1

Hi Olufemi Olaoye,

Thanks for joining the call.

As discussed, when a VMSS scales out an instance, the new VM may not be aware of what exactly happened on the parent VM. There are Uniform and Flexible orchestration modes in the VMSS. In Uniform mode, the VMs are identical and are used for stateless workloads whereas Flexible mode VMs are ideal for stateful workloads and the VMSS does horizontal scaling. In horizontal scaling, VMSS increases the number of VMs, not their size.

Please let me know if you have any questions.

If it was helpful, please click "Upvote" on this post and "Accept" the answer.

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