VMs on different Fault Domains in a specific AZ

Question

VMs on different Fault Domains in a specific AZ

Sinan Petrus Toma 6

Hello everyone,

I see, when choosing Availability Sets, the VMs will be automatically deployed across a number (3) of fault domains.
However, in multi-AZ regions, I can't see any option to select an AZ and even find the information, in what AZ those VMs (the Availability Set) are.
Sure, in multi-AZ regions, we would choose "Availability Zones" as an availability option, not Availability Sets.

But I need both.

Place three VMs on different Fault Domains in the same AZ, and
Choose the AZ for those 3 VMs (or at least have the information about that AZ afterward).

Is this possible in Azure?

Regards,
Sinan

2 answers

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Answer 1

Alan Kinane 16,951 MVP Volunteer Moderator

If you are using Availability Zones then you are automatically using different fault and update domains as you are actually hosting your VMs across multiple datacentres so naturally these VMs cannot be on the same physical hosts. Therefore these two options are mutually exclusive, you can choose one or the other with availability zones being the more resilient option as this not only spreads your VMs across fault and updates domains but also across different datacentres.

In your example you should deploy your 3 VMs across three separate availability zones for best resiliency otherwise if you deploy to an availability set they will all be in the same datacentre but on separate fault/update domains.

You also get a higher SLA on Azure VMs based on which option you choose.

Sinan Petrus Toma 6 Reputation points

2022-02-23T17:12:18.183+00:00

Thank you Alan for your answer. Yes, this is all clear.
My use case requires both, VMs spread across AZs and fault domains in the same AZ, e.g.:
VM1, VM2, and VM3 are all in AZ1, on different hardware (different fault domains), and
VM4, VM5, and VM6 are all in AZ3, on different hardware (different fault domains).
So, when you say Availability Zones and Availability Sets are mutually exclusive, I assume the above configuration is not possible. Could you confirm, please?

PS: when choosing Availability Sets, I'm making sure that VM1, VM2, and VM3 are on different hardware, but in the same data center (same AZ), right? Any way to figure out WHAT AZ is that? Maybe an OS command on the host itself or looking at some IDs etc.? As per Azure Portal, this information is not disclosed. When I choose Availability Set, I get Availability Zone=None. But the VMs MUST be in an AZ. The question is just which one?
Alan Kinane 16,951 Reputation points MVP Volunteer Moderator

2022-02-23T17:23:41.38+00:00

Correct, you can only choose to use either availability sets or availability zones. The concept is to provide an extra level of resiliency for VM instances running the same workload, e.g. 2 domain controllers, 3 databases servers etc. So if you had 6 VMs, the best option for you would be to deploy 2 VMs into each of the 3 availability zones.

As far as I know there is no way to see which zone your VM is deployed into if you do not choose this option. Azure automatically chooses one of the zones for you but this is not disclosed or at least I have never been able to discover this.
Sinan Petrus Toma 6 Reputation points

2022-02-23T18:02:22.653+00:00

Thank you for confirming.
It is not about 6 VMs for the same workload. But understood that it is not possible to choose AZ AND Fault Domain within the AZ at the same time.
PS: in other clouds like AWS and Oracle Cloud, this is possible, and actually very easy and straightforward.

Answer 2

Sinan Petrus Toma 6

I got a way how to create VMs in an Availability Set (of different fault domains) while knowing the AZ at the same time:

Create a PPG and create VM1 in AZ1 and PPG to scope the PPG --> PPG is scoped to AZ1
Create an Availability Set and assign it to the scoped PPG
Create VM2 and VM3 in Availability Set --> VM2 and VM3 on different Fault Domains (different Hardware) in AZ1

As described here:
https://learn.microsoft.com/en-us/azure/virtual-machines/workloads/sap/sap-proximity-placement-scenarios

Alan Kinane 16,951 Reputation points MVP Volunteer Moderator

2022-03-21T18:52:42.963+00:00

That's interesting, thanks for sharing.

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VMs on different Fault Domains in a specific AZ

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