VMSS Flex Fault Domains

Question

My understanding is that VMSS Flex distributes VMs over Fault Domains and that Flexible Orchestration is now the recommended approach to using VMSS. Although AvailabilitySets still exist, is the recommended approach to use VMSS Flex instead (or the alternative question is can you replace an AvailabilitySet with VMSS Flex). I have been trying to confirm this with a simple experiment.

I created a scale set using the following
az vmss create --resource-group myvm-resource-gp --name myScaleSet --orchestration-mode flexible --platform-fault-domain-count 2

Then added a VM
az vm create --name myVM1 --resource-group myvm-resource-gp --vmss myScaleSet --size Standard_DS1_v2 --image UbuntuLTS --vnet-name myVnet --subnet mySubnet --admin-username azureuser --generate-ssh-keys

I thought I would try to determine the fault domain using
az vm list --resource-group myvm-resource-gp --show-details
But the platformFaultDomain is set to null. I also tried the Azure Resource Graph approach with no success.
The experiment is probably flawed since we should not be interested in the fault domain a VM is using, hence the above question.

Answer 1

Hi @MikeD

Thank you for reaching out to Microsoft Q &A Platform .

I have reviewed your sample use case , And observed one parameter is missing in your command :

Here are my examples:
az vm create --name myVM4 --resource-group vmsstest --vmss myScaleSet --size Standard_DS1_v2 --image UbuntuLTS --vnet-name myVnet --subnet mySubnet --admin-username azureuser --generate-ssh-keys --platform-fault-domain 1

az vm create --name myVM5 --resource-group vmsstest --vmss myScaleSet --size Standard_DS1_v2 --image UbuntuLTS --vnet-name myVnet --subnet mySubnet --admin-username azureuser --generate-ssh-keys --platform-fault-domain 0

If you add the FD parameter you will see that the VM lands in the specified FD .

az vm list --resource-group vmsstest gives me the correct results

Thanks & Regards,
Pradeep

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