According to AWS document, if account a in a consolidated billing organization bought an RI in an AZ (say, us-east-1a), and account b in the same organization ran an instance with matching attributes of the RI (the same tenancy, platform, AZ, instance type, and instance size), then that instance can benefit from the RI discount.
My question is that since different accounts map the same AZ name to different physical locations, and I guess zonal RI should be reserved in a physical location, how can account b benefit from the RI bought by account a?
I googled for the answer but got no clue.
That’s a really good question. Yes, in consolidated billing organizations, you can make use of RI’s across accounts if Reserved Instances Sharing is enabled. However, the second account must launch in the same AZ by name and it seems that AWS does not honor the "reservation" aspect of a Zonal RI if it’s consumed by another account…likely because of the different physical AZ locations.