SAA-C03 practice question: EC2 purchasing options (1 of 3)
Design Cost-Optimized Architectures A free question from our original SAA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.
A genomics research team runs a batch pipeline every night from 22:00.
It needs about 400 vCPUs for roughly 6 hours and must finish by 07:00, which leaves about 3 hours of slack.
Jobs save a checkpoint every 10 minutes and can resume on any instance. The pipeline runs well on many compute optimized and general purpose instance types, and the capacity is not needed during the day.
Which purchasing approach gives the LOWEST cost while meeting the deadline?
Choose one answer.
AA. Use a Spot-only Auto Scaling group spanning many instance types and all Availability Zones, with the price-capacity-optimized strategy.
BB. Use a Spot-only Auto Scaling group with one instance type in one Availability Zone, with the lowest-price allocation strategy.
CC. Buy a 1-year Compute Savings Plan sized for 400 vCPUs of On-Demand usage, and run the pipeline on On-Demand Instances each night.
DD. Run the pipeline on On-Demand Instances that a scheduled scaling action launches at 22:00 nightly and ends when the jobs finish.
Show the answer and explanation
Answer: A
A. Right: Correct. Spot Instances give the deepest discount, and checkpointing means an interruption costs at most a few minutes of work. Drawing on many instance types and zones with price-capacity-optimized picks pools with spare capacity, which lowers the chance of interruptions eating into the slack. Learn more: Best practices for Amazon EC2 Spot (opens in a new tab)
C. Wrong: A Savings Plan is an hourly commitment, and each hour's commitment cannot be carried over to another hour. The pipeline runs about 6 hours a day, so most of the commitment would be paid for and unused. Spot pricing is cheaper for interruptible night-time work. Learn more: Understanding how Savings Plans apply to your usage (opens in a new tab)
D. Wrong: This avoids paying for idle daytime capacity and never risks interruption, but every hour is charged at full On-Demand price. The jobs checkpoint and have slack, so they can safely use Spot capacity at a much lower price. Learn more: Amazon EC2 billing and purchasing options (opens in a new tab)
Explanation
Checkpointed, restartable jobs with slack in the schedule are a good fit for Spot Instances. How Spot is requested matters: being flexible across many instance types and Availability Zones, with the price-capacity-optimized strategy, reduces interruptions and capacity shortfalls that could threaten the deadline. Commitments such as Savings Plans suit steady round-the-clock use, not 6 hours a night.
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