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Convertible caveat (#442)

Corey Quinn 4 years ago
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@@ -1929,6 +1929,7 @@ Billing and Cost Management
- Consider using Reserved Instances when you can predict your longer-term compute needs and need a stronger guarantee of compute availability and continuity than the (typically cheaper) Spot market can provide. However be aware that if your architecture changes your computing needs may change as well so long term contracts can seem attractive but may turn out to be cumbersome.
- There are two types of Reserved Instances - [Standard and Convertible]( If you purchase excess Standard Reserved Instances, you may offer to “sell back” unused Reserved Instances via the [Reserved Instance Marketplace](, this allows you to potentially recoup the cost of unused EC2 compute instance hours by selling them to other AWS customers.
- Instance reservations are not tied to specific EC2 instances - they are applied at the billing level to eligible compute hours as they are consumed across all of the instances in an account.
- 📜There have been scattered reports of Convertible RI purchases needing to be exercised in a block-- namely, if you buy five convertible RIs in one purchase, you can't convert just two of them. Reach out to your account manager for clarification if this may impact you.
- **Hourly billing waste:** EC2 instances are [billed in instance-hours]( — rounded up to the nearest full hour! For long-lived instances, this is not a big worry, but for large transient deployments, like EMR jobs or test deployments, this can be a significant expense. Never deploy many instances and terminate them after only a few minutes. In fact, if transient instances are part of your regular processing workflow, you should put in protections or alerts to check for this kind of waste.
- If you have multiple AWS accounts and have configured them to roll charges up to one account using the “Consolidated Billing” feature, you can expect *unused* Reserved Instance hours from one account to be applied to matching (region, availability zone, instance type) compute hours from another account.
- If you have multiple AWS accounts that are linked with Consolidated Billing, plan on using reservations, and want unused reservation capacity to be able to apply to compute hours from other accounts, you’ll need to create your instances in the availability zone with the same *name* across accounts. Keep in mind that when you have done this, your instances may not end up in the same *physical* data center across accounts - Amazon shuffles availability zones names across accounts in order to equalize resource utilization.