AWS EC2 Windows, reserved instances

Question

I was wondering what my options are for reserved instances for Windows machines.

Say I have a machine as m5.2xlarge, which will run for the coming year. I could buy a reserved instance for that. However, I understood that I cannot modify the instance type/size for windows reserved instances. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modifying.html#ri-modification-limits

So, I could scale the server to a m5.4xlarge when that is needed but cannot modify the reserved instance accordingly.

I did understand correctly that reserved instance can also be applied to cover half of the server? So would myreservation at least cover half of it then?

Should I take convertable instances instead? Can I convert an m5.2xlarge Windows reserved instance to an m5.4xlarge during the reservation period?

Thanks!

Answer 1

This is a tricky one, as much of the flexibility afforded to reserved instances doesn't apply to Windows instances, as in your case.

The only true answer to this question is based on your judgement of your growing requirements: How likely is it that you're going to need an instance of double the size within the next year? If it's very likely, then perhaps a convertible reserved instance is for you - but the discount will be less than if you use a Standard Reserved instance.

If you're creating a new application and aren't sure, I'd recommend running some tests using spot instances to get a gauge for required size, or use an instance on-demand for a short time until you are certain of your requirements - if you want to purchase a Standard reservation to get the maximum discount.

You might also want to consider Scheduled Reserved Instances: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-scheduled-instances.html if your workloads are likely onto to be within standard office hours, say 9am-5pm during the week, which would almost certainly be cheaper than a 24x7 reservation of any other type if you're not going to use it. Remember with reserved instances - if you don't use them all day every day, you're losing value.