Spot Instances are spare compute capacity that cloud providers offer at a lower price than On-Demand Instances. This can result in significant cost savings when performing internal tests and debugging in development or QA environments. The performance of On-Demand and Spot Instances of the same instance type is similar.

Spot Instances are most beneficial when the frequency of interruptions is under 5%. Use the Spot Instance advisor on AWS to see a list of instances with different levels of interruptions.

The following chart shows the potential savings between On-Demand and Spot Instances. The chart also shows the differences in savings with different levels of frequency of interruptions:

In the chart, you can see that when the frequency of interruption is below 5%, Spot Instances can save you nearly 50% on the total cost compared to On-Demand Instances. However, when the frequency of interruption exceeds 20%, your savings drops to 36%.

When you use Spot Instances, you set a Spot Instance price ratio. The Spot Instance price ratio is the maximum price you will pay for Spot Instances as a percentage of the On-Demand Instance price. For example, if On-Demand Instances cost $0.68 an hour and you set the Spot Instance price ratio to 50, you will pay the current Spot Instance price as long as the price is $0.34 an hour or less.

The Secure Agent always creates a number of On-Demand worker nodes equal to the minimum number of worker nodes that you configure. When you enable Spot Instances and the cluster scales up, the agent tries to create additional worker nodes on Spot Instances up to the maximum number of worker nodes. If Spot Instances are not available or cost more than the maximum price you set, the cluster uses On-Demand Instances for the worker nodes.

For example, if you set the minimum number of worker nodes to 5 and the maximum to 8, the agent creates 5 nodes on On-Demand Instances and tries to create 3 nodes on Spot Instances. If you set the maximum number of worker nodes equal to the minimum, the cluster uses only On-Demand Instances.

If your cloud provider interrupts a Spot node that is running an

advanced job

, the agent uses On-Demand nodes to complete the job.