Unable to start Spot Instance VM after Resize

Question

I am unable to start my spot instance VM after resizing it from HHB120rs_v2 to HB120rs_v3, with the error:

Failed to start virtual machine 'VM2'. Error: Allocation failed. VM(s) with the following constraints cannot be allocated, because the condition is too restrictive. Please remove some constraints and try again. Constraints applied are:

• Low Priority VMs
• Networking Constraints (such as Accelerated Networking or IPv6)
• Preemptible VMs (VM might be preempted by another VM with a higher priority)
• VM Size

This is all done within the US East region. Interestingly, when resizing, HB120rs_v3 is not shown under the H-Series category, but instead under the Other category. It previously appeared under the H-Series category, when I was able to start it correctly. If I change the size back to HB120rs_v2 I am able to start the VM.

This is not a quota issue as I currently only have 1 VM and the VM was deallocated at the time of resizing. I have also previously successfully started this VM with the HB120rs_v3 size about 4 weeks earlier.

My questions are:

1. How can I determine the specific cause of the start failure?
2. What is the significance of the VM size being shown under the Other category?

Answer 1

We cannot change the generation of a VM after we create it

How can I determine the specific cause of the start failure?

The reason for VM start failure is the last operation that was run on the VM failed after the input was accepted

As you are trying to change the gen2 VM to gen3 which is not supported, this might be the reason for the start failure

What is the significance of the VM size being shown under the other category?

When resizing, HB120rs_v3 is not shown under the H-Series category because the hardware cluster currently used for HB120rs_v2 VM does not support the HB120rs_v3 VM

So HB120rs_v3 is shown under other category but not under H-series

Answer 2

The error message should generally be taken at face value. It means that the platform could not satisfy all the given allocation (placement) constraints together. If you remove one or more constraints (some of which are expressed indirectly as feature/capability specifications in the VM model), the allocation may succeed. For a standalone VM (not in an Availability Set or a VMSS), common highly restrictive allocation constraints are IPv6 and Accelerated Networking, often in relation to the VM size. (But you can be certain the VM size by itself is compatible with Accelerated Networking; otherwise the request would've been rejected earlier, with a more specific error.)

'Spot' priority (referred to as "Preemptible VMs" in the error message) can also be restrictive since there must be ample excess capacity available to allow Spot deployments in a region/zone. A good resource for assessing Spot availability is the eviction history: https://docs.microsoft.com/en-us/azure/virtual-machines/spot-vms#pricing-and-eviction-history.