[jedberg]

This article talks about the financial pros and cons of reserved instances, but not the important operational benefit:

You will always get an instance when you need it, and never get an error about not having enough instances of a certain type.

This is critical if you are using autoscaling, especially during an outage. If you are shifting your workload from one region to another at the same time as everyone else, if you have reservations and they don't, you'll get the instances before anyone else.

It breaks the whole "pay as you go" cloud model, but as long as you can find batch jobs to fill those reserved instances 1/3 of the time, you'll still come out ahead.

[scarytom]

That used to be true, when AWS only sold AZ-specific RIs, but for a long time now AWS have (by default) sold region-level RIs instead, and these do not include a capacity reservation. You can now purchase Capacity Reservations [0] on their own.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capa...

[jsmith45]

That actually depends.

Reserved instances have one form with zero operation benefits: Regional Reserved Instances. These are 100% purely billing constructs.

They have zonal Reserved Instances which do reserve capacity.

Finally they have pure capacity reservations. These cost full price. Esentially you are paying for an instance even if it is not running, but it does guarentee it will be there if you need it. but it also has no comittments.

However combining the capacity reservation with a savings plan allows you to basically recreate the net effect of a zonal Reserved Instance but with additional flexibility.

Regional Reserved Instance's discount will also apply to reserved capacity if there were not enough actual instances to cover the Reserved Instances.

[rescbr]

> However combining the capacity reservation with a savings plan allows you to basically recreate the net effect of a zonal Reserved Instance but with additional flexibility.

This is something that has never crossed my mind. TIL.