Azure VMs, Monitoring and Load Balancing

Question

Azure VMs, Monitoring and Load Balancing

jbassking 106

I have two Windows VMs behind a load balancer with monitoring setup on both VMs. When checking the CPU utilization, I notice that the load isn't evenly balanced. Here's what I'm seeing. Does this look normal for a 24 hour period? When checking for a shorter time period, it seems like the balancing isn't even.

These are Standard B2s VMs. They're hosting an API. Looking at the most recent server requests, we're getting 600k-750k in the last 7 days. We're using the Azure Basic LB with 2 http and 2 https backend pools.

24 Hours

1 Hour

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Answer 1

msrini-MSFT 9,291 Microsoft Employee

@jbassking ,

Azure Load balancer does hash based load balancing. Here are few things to keep in mind when you perform Load balancing with Azure Load Balancer:

Make sure you select the Session Persistence as None to load balance traffic based on 5 tuple.
Make sure that the Second VM is responding to the probes. If the probes are down for 2nd VM, then the Load balancer will not forward the traffic to that VM.
CPU utilization is not always the right method to check the load is being balanced. There might be multiple factors associated. But I get that having less load for majority of time from your screenshot hints no traffic is sent to that VM during that time.

With the above suggestion, I think you should be able to get your Load balancing on track. If you have any questions, let me know.

Regards,
Karthik Srinivas

jbassking 106 Reputation points

2022-01-15T06:12:29.583+00:00

Thanks for the reply @msrini-MSFT . The Session Persistence is set to None on both servers. I'm not sure what you mean by "responding to probes". Since the issue isn't always server 1 or server 2, I would assume that they're both configured the same. But just to make sure, how do I check to see if they're both responding to the probes?
msrini-MSFT 9,291 Reputation points Microsoft Employee

2022-01-16T16:55:17.173+00:00

Looks like load balance is happening but not 50% and 50% at all the time. It is expected with Azure Load balancer. The pattern that you are seeing is because, with the same session more traffic is sent. So that will be hitting to the same server. Once new session is created, it will again be hashed and it might end up in the same server. It is not equal load balancing that you can get from Azure Load balancer.

Answer 2

Devaraj G 2,096 Volunteer Moderator

Hi,

Azure Load Balancer doesn't support true round robin load balancing but supports a hash based,
Check your Flow Distribution tab to get better understanding and this will help you visualize and manage the number of flows your backend instances are receiving and producing. See if there is lots of uneven loads.

I think what you seeing is normal. Check out this article to understand the traffic flow for hash based.
https://learn.microsoft.com/en-us/azure/load-balancer/distribution-mode-concepts

jbassking 106 Reputation points

2022-01-18T02:59:03.977+00:00

I'm not really sure how to interpret the Flow Distribution but I'll look into it more.
Devaraj G 2,096 Reputation points Volunteer Moderator

2022-01-21T04:35:45.977+00:00

Thanks. Please let us know for any other queries.

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Azure VMs, Monitoring and Load Balancing

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