A company runs jobs that can finish at any time. Currently the jobs use multiple EC2 On-Demand Instances and usually run just under two hours. If a job fails it must restart from the beginning. Which approach is the most cost-effective to run these jobs?

Spot Instances with a defined duration (spot blocks) are the most cost-effective solution here. They offer significant cost savings compared to On-Demand instances, and the defined duration ensures that the instances will not be interrupted before the job's expected completion time (under two hours), preventing costly restarts. Buying Reserved Instances is less flexible and cost-effective for jobs that run intermittently and for short durations, as the commitment period might exceed the actual usage. A single one-off Spot Instance is cheaper than On-Demand but carries a higher risk of interruption, which would force a restart and negate cost savings. Running a mix of On-Demand and Spot Instances adds complexity and doesn't fully leverage the cost benefits of Spot Instances for predictable short-duration tasks.