RI are a billing concept (discounted rates for long term commitment).

Dedicated capacity exists, but it’s different (compute reservation groups or dedicated hosts).

You can combine CRG/DH with RI for the desired effect, although IMO it’s a bit confusing.

(Azure employee)

It's a billing mechanism. You pay less if you guarantee use. Sadly, they don't guarantee availability of things to use :)

Yup, I'm aware of reserved instances (from an AWS PoV) but I always assumed they were, at least theoretically, well, reserved!

Reminds me of the classic Seinfeld car reservation bit: https://m.youtube.com/watch?v=4T2GmGSNvaM

Great bit! Same with airline overbooking ;)

On AWS instances are only reserved if you reserve a specific instance type in a specific zone. Reservations across multiple zones or savings plans don't reserve capacity.

In the AWS context, they are, in fact. That's the original point of them - so during big AZ failures your reserved instances had first dibs on the available capacity.

The billing thing became more of the point as big AZ failures are so rare.

Non-zonal instance reservations do not reserve capacity. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capa...

Well, yeah. But the zonal ones still do.

With dedicated tenancy AWS Reserved Instances are physically reserved for you.