Managing costs in public cloud environments, such as AWS, Azure, and GCP, is a complex challenge. One appealing solution is the use of spot instances. Also known as "spot VMs" on Azure and GCP, these instances offer cheaper compute power with the caveat that they might be interrupted with short notice (120s on AWS and GCP, 30s on Azure).
In theory, if your workload can handle interruptions, you should always use spot instances. It’s also recommended to expand the search of spot instances across all of the availability zones in a region to secure the best price. However, in practice, the cost of compute isn’t the sole consideration. Companies that extensively use spot instances may find themselves with a large, unexpected networking bill due to the traffic between the spot instance and other services.
A Closer Look at the Numbers:
For a m5.large workload that runs for 5 minutes:
On-demand price: $0.096/hr
Average spot instance price: $0.061/hr
Price in availability zone us-east-1b: $0.0488/hr
Should you use us-east-1b for your workload? The savings here would be $0.0122/hr, or approximately $0.001 for the 5-minute run. In AWS, the list price for data transfer in us-east-1 is $0.01/GB. If this workload is transferring more than 100 MB during its lifetime, then the switch to us-east-1b will cost more than you are saving.
One might ask - how do I know how much data is being transferred? In some cases, it’s fairly simple to benchmark based on a few examples. But if you want a deeper view into your networking costs, feel free to reach out at datatransfer@underspend.com. We have a powerful tool that provides a full picture of who in your network is talking to whom, and how much it’s costing you.
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