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We have bought some reserved EC2 instances in different availability zones in eu-west-1 region. When launching a new instance, via API or AWS control panel, we do usually set "No preference" as the availability zone.

In case of we have an "unused" reserved instance in an availability zone, does AWS still prefer that very zone even "No preference" is selected when launching the instance? Or, should we explicitly define the availability zone to match the inactive instance reservation?

I tried to look for the answer from the AWS documentation, but without luck yet:

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up vote 1 down vote accepted

Yes, if you have reserved instances purchased in us-east-1b, for example, then you explicitly want to specify us-east-1b at launch. RI's are purely a backend billing thing that function like an invoice credit. The EC2 instances themselves have no awareness of whether they're an RI or not.

The way RI's are handled is actually one of my top five complaints about AWS. I'm hoping that they improve on this soon.

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Thank you for your answer! Just wondering, do you have any references that would confirm this behavior? –  Ville Mattila Mar 14 '13 at 23:35
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