Differance between Guest OS metrics and Host OS metrics in Azure VMSS

Question

I am setting up auto-scale rules in Azure VMSS. I can choose from two set of metrics, Host and Guest. As per the documentation, both are emitted from same VMs.

My tech stack on each VM

 - [Tomcat 9.0.30][2]
 - [APR 1.7.0][2]
 - 9 [Servlets][2].

Planning on setting up these autoscale rules

 - Add 1 machine when CPU%>70 for 10 minutes.
 - Add 2 machines when CPU%>80 for 10 minutes.
 - Remove 1 machine when CPU%<60 for 10 minutes.

I have following questions.

1. What's the difference between Host and Guest metrics. Since both are coming from the same VM.
2. Which ones to choose for more accurate scaling of tomcat.
3. Apart from monitoring CPU, is there a more efficient way to scale tomcat in Azure VMSS, like system memory.

I researched and found some good links, but couldn't get a definitive answer. Some of the links I found are:

• https://pediaa.com/what-is-the-difference-between-host-and-guest-operating-system/
• https://learn.microsoft.com/en-us/azure/azure-monitor/app/azure-vm-vmss-apps
• https://mykloud.wordpress.com/2018/02/11/virtual-machine-scale-set-vmss/

Answer 1

What's the difference between Host and Guest metrics.

• A host operating system is a software that is installed on a computer and communicates with the hardware. A guest OS, on the other hand, is software that is installed in a virtual machine.
• The host OS runs on the hardware directly. The guest OS, on the other hand, runs on a virtual machine.
• It's possible that the host Operating System may be single . The guest OS, on the other hand, can be single or numerous.
• The host operating system interacts with the hardware. The guest operating system interacts with the virtual machine.

Which ones to choose for more accurate scaling of tomcat.

• We can scale up or down using host-level metrics, but we won't be able to do that with guest-level metrics.
• Guest OS metrics include Performance counters which monitor guest CPU % or memory consumption, which are commonly used for autoscaling.

Is there a more efficient way to scale tomcat in Azure VMSS, like system memory.

• You may use Azure Monitor for VMs to automate the collection of essential CPU, memory, disks, and network performance counters from the VMs in your scale set. It also comes with additional monitoring features and pre-built visualisations to help you focus on the performance and reliability of your scale sets.

For more information please refer these links : What are virtual machine scale sets , Guest OS and host OS metrics , Overview of autoscale with Azure virtual machine scale sets