Spot Priority Mix for high availability and cost savings

Article
08/22/2024

Applies to: ✔️ Flexible scale sets

Azure allows you to have the flexibility of running a mix of uninterruptible standard VMs and interruptible Spot VMs for Virtual Machine Scale Set deployments. You're able to deploy this Spot Priority Mix using Flexible orchestration to easily balance between high-capacity availability and lower infrastructure costs according to your workload requirements. This feature allows you to easily manage your scale set capability to achieve the following goals:

Reduce compute infrastructure costs by applying the deep discounts of Spot VMs
Maintain capacity availability through uninterruptible standard VMs in the scale set deployment
Provide reassurance that all your VMs won't be taken away simultaneously due to evictions before the infrastructure has time to react and recover the evicted capacity
Simplify the scale-out and scale-in of compute workloads that require both Spot and standard VMs by letting Azure orchestrate the creation and deletion of VMs

Limitations

Spot Priority Mix is not supported with singlePlacementMode enabled on the scale set.

Configure your mix

You can configure a custom percentage distribution across Spot and standard VMs. The platform automatically orchestrates each scale-out and scale-in operation to achieve the desired distribution by selecting an appropriate number of VMs to create or delete. You can also optionally configure the number of base standard VMs you would like to maintain in the Virtual Machine Scale Set during any scale operation.

The eviction policy of your Spot VMs follows what is set for the Spot VMs in your scale set. Deallocate is the default behavior, wherein evicted Spot VMs move to a stop-deallocated state. Alternatively, the Spot eviction policy can be set to Delete, wherein the VM and its underlying disks are deleted.

Scale-In Policy

When using Spot Priority Mix, your scale-in policy for the scale set will operate to try to maintain the percentage split of the Spot and Standard VMs in your scale set. Spot Priority Mix will determine if Spot or Standard VMs need to be removed during scale-in actions to maintain your percentage split, rather than deleting the oldest or newest VM.

ARM Template

You can set your Spot Priority Mix by using an ARM template to add the following properties to a scale set with Flexible orchestration using a Spot priority VM profile:

"priorityMixPolicy": {
    "baseRegularPriorityCount": 0,
    "regularPriorityPercentageAboveBase": 50
},

Parameters:

baseRegularPriorityCount – Specifies a base number of VMs that are standard, Regular priority; if the Scale Set capacity is at or below this number, all VMs are Regular priority.
regularPriorityPercentageAboveBase – Specifies the percentage split of Regular and Spot priority VMs that are used when the Scale Set capacity is above the baseRegularPriorityCount.

You can set your Spot Priority Mix in the Spot tab of the Virtual Machine Scale Sets creation process in the Azure portal. The following steps instruct you on how to access this feature during that process.

Log in to the Azure portal.
In the search bar, search for and select Virtual Machine Scale Sets.
Select Create on the Virtual Machine Scale Sets page.
In the Basics tab, fill out the required fields, select Flexible as the Orchestration mode, and select the checkbox for Run with Azure Spot discount.
In the Spot tab, select the check-box next to Scale with VMs and Spot VMs option under the Scale with VMs and discounted Spot VMs section.
Fill out the Base VM (uninterruptible) count and Instance distribution fields to configure your percentage split between Spot and Standard VMs.
Continue through the Virtual Machine Scale Set creation process.

You can set your Spot Priority Mix using Azure CLI by setting the priority flag to Spot and including the regular-priority-count and regular-priority-percentage flags.

az vmss create -n myScaleSet \
		-g myResourceGroup \
		--orchestration-mode flexible \
		--regular-priority-count 2 \
		--regular-priority-percentage 50 \
		--orchestration-mode flexible \
		--instance-count 4 \
		--image Ubuntu2204 \
		--priority Spot \
		--eviction-policy Deallocate \
		--single-placement-group False \

You can set your Spot Priority Mix using Azure PowerShell by setting the Priority parameter to Spot and including the BaseRegularPriorityCount and RegularPriorityPercentage parameters.

$vmssConfig = New-AzVmssConfig `
            -Location "East US" `
            -SkuCapacity 4 `
            -SkuName Standard_D2_v5 `
            -OrchestrationMode 'Flexible' `
            -EvictionPolicy 'Delete' `
            -PlatformFaultDomainCount 1 `
            -Priority 'Spot' `
            -BaseRegularPriorityCount 2 `
            -RegularPriorityPercentage 50;

New-AzVmss `
            -ResourceGroupName myResourceGroup `
            -Name myScaleSet `
            -VirtualMachineScaleSet $vmssConfig;

Updating your Spot Priority Mix

Should your ideal percentage split of Spot and Standard VMs change, you can update your Spot Priority Mix after your scale set has been deployed. Updating your Spot Priority Mix will apply for all scale set actions after the change is made, existing VMs will remain as is.

You can update your existing Spot Priority Mix in the Configuration tab of the Virtual Machine Scale Set resource page in the Azure portal. The following steps instruct you on how to access this feature during that process. Note: in Portal, you can only update the Spot Priority Mix for scale sets that already have Spot Priority Mix enabled.

Navigate to the specific virtual machine scale set that you're adjusting the Spot Priority Mix on.
In the left side bar, scroll down to and select Configuration.
Your current Spot Priority Mix should be visible. Here you can change the Base VM (uninterruptible) count and Instance distribution of Spot and Standard VMs.
Update your Spot Mix as needed.
Press the Save button to apply your changes.

You can update your Spot Priority Mix using Azure CLI by updating the regular-priority-count and regular-priority-percentage parameters.

az vmss update --resource-group myResourceGroup \
        --name myScaleSet \
        --regular-priority-count 10 \
        --regular-priority-percentage 80 \

You can update your Spot Priority Mix using Azure PowerShell by updating the BaseRegularPriorityCount and RegularPriorityPercentage parameters.

$vmss = Get-AzVmss `
        -ResourceGroupName "myResourceGroup" `
        -VMScaleSetName "myScaleSet"

Update-AzVmss `
        -ResourceGroupName "myResourceGroup" `
        -VirtualMachineScaleSet $vmss
        -VMScaleSetName "myScaleSet" `
        -BaseRegularPriorityCount 10 `
        -RegularPriorityPercentage 80;

Examples

The following examples have scenario assumptions, a table of actions, and walk-through of results to help you understand how Spot Priority Mix configuration works.

Some important terminology to notice before referring to these examples:

sku.capacity is the total number of VMs in the Virtual Machine Scale Set
Base (standard) VMs are the number of standard non-Spot VMs, akin to a minimum VM number

Scenario 1

The following scenario assumptions apply to this example:

sku.capacity is variable, as Autoscale will add or remove VMs from the scale set
Base (standard) VMs: 10
Extra standard VMs: 0
Spot priority VMs: 0
regularPriorityPercentageAboveBase: 50%
Eviction policy: Delete

Action	sku.capacity	Base (standard) VMs	Extra standard VMs	Spot priority VMs
Create	10	10	0	0
Scale out	20	10	5	5
Scale out	30	10	10	10
Scale out	40	10	15	15
Scale out	41	10	15	16
Scale out	42	10	16	16
Scale in - Evict-Delete (all Spot instances)	26	10	16	0
Scale out	30	10	16	4
Scale out	42	10	16	16
Scale out	44	10	17	17

Example walk-through:

You start out with a Virtual Machine Scale Set with 10 VMs.
- The sku.capacity is variable and doesn't set a starting number of VMs. The Base VMs are set at 10, thus your total starting VMs are just 10 Base (standard) VMs.
You then scale-out 5 times, with 50% standard VMs and 50% Spot VMs.
- Note, because there's a 50/50 split, in the fourth scale-out, there's one more Spot VM than standard VM. Once it's scaled out again (5th scale-out), the 50/50 balance is restored with another standard VM.
You then scale in your scale set with the eviction policy being evict-delete, which deletes all the Spot VMs.
With the scale-out operations mentioned in this scenario, you restore the 50/50 balance in your scale set by only creating Spot VMs.
By the last scale-out, your scale set is already balanced, so one of each type of VM is created.

Scenario 2

The following scenario assumptions apply to this example:

sku.capacity is variable, defined by autoscaler; starting with 20
Base (standard) VMs: 10
Extra standard VMs: 2
Spot priority VMs: 8
regularPriorityPercentageAboveBase: 25%
Eviction policy: Deallocate

Action	sku.capacity	Base (standard) VMs	Extra standard VMs	Spot priority VMs
Create	20	10	2	8
Scale out	50	10	10	30
Scale out	110	10	25	75
Scale In: Stop-Deallocate (10 instances)	100	10	25	75 (65 running VMs, 10 Stop-Deallocated VMs)
Scale out	120	10	27	83 (73 running VMs, 10 Stop-Deallocated VMs)

Example walk-through:

With the initial creation of the Virtual Machine Scale Set and Spot Priority Mix, you have 20 VMs.
- 10 of those VMs are the Base (standard) VMs, 2 extra standard VMs, and 8 Spot priority VMs for your 25% regularPriorityPercentageAboveBase.
- Another way to look at this ratio is you have 1 standard VM for every 4 Spot VMs in the scale set.
You then scale out twice to create 90 more VMs; 23 standard VMs and 67 Spot VMs.
When you scale in by 10 VMs, 10 Spot VMs are stop-deallocated, creating an imbalance in your scale set.
Your next scale out operation creates another 2 standard VMs and 8 Spot VMs, bringing you closer to your 25% above base ratio.

Troubleshooting

If Spot Priority Mix isn't available to you, be sure to configure the priorityMixPolicy to specify a Spot priority in the virtualMachineProfile. Without enabling the priorityMixPolicy setting, you won't be able to access this Spot feature.

FAQs

Q: I changed the Spot Priority Mix settings, why aren't my existing VMs changing?

Spot Priority Mix applies for scale actions on the scale set. Changing the percentage split of Spot and Standard VMs won't rebalance existing scale set. You'll see the actual percentage split change as you scale the scale set.

Q: Is Spot Priority Mix enabled for Uniform orchestration mode?

Spot Priority Mix is only available on Virtual Machine Scale Sets with Flexible orchestration mode.

Q: Which regions is Spot Priority Mix enabled in?

Spot VMs, and therefore Spot Priority Mix, are available in all global Azure regions.

Next steps

Learn more about Spot virtual machines

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