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I am a little bit confused with the way EC2 pricing works with reserved instances. It is my understanding that EC2 reserved instances are just a way to price instances.

For my application I need to randomly create a new instance and terminate the current, but that has to happen with no major delays as sometimes happen with spot instances where you have to bid the right price or wait for the price to drop to your level.

In the case of purchasing a reserved instance that would mean there will always be an instance for me?

Any clarification on this will be appreciated. Thanks

JordanBelf
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2 Answers2

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Unless there is some other system issue. On demand instances always launch. A reservation is simply a billing feature and will bill a matching on demand instance at the reservation price.

datasage
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  • It's technically possible for there to be no available instances to launch of a given class in a given availability zone. If you have more reserved instances than running instances of that class in that zone, the reservation should prevent that condition from impacting you... but such a scenario seems uncommon. Spot instance availability is never guaranteed, since spot is defined as "surplus capacity." Demand instances are not technically guaranteed to always always always be launchable without a reserved instance, but I've never encountered that condition. – Michael - sqlbot Feb 03 '15 at 11:24
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Reservations have two parts to them, a cost savings, and an capacity "guarantee." I'm putting that in quotes because there's no way to completely guarantee capacity.

So, for instance, if you have a reservation for a m3.xlarge running Linux in us-east-1a, and there's a run on that instance type in that AZ, you'll be given priority over someone requesting that instance without a reservation.

kaffenated
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