Does the available EC2 instances types vary over availablity zones (in the same region)?

Question

Amazon launched a third availability zone in Frankfurt: eu-central-1c

From what I see, the third availability zone does not support all instance types that are available in the existing zones (eu-central-1a and eu-central-1b).

I always assumed that the available instance types may differ over regions but are identical over availability zones within the same region. From what I see that is not the case.

Is it true that you if an EC2 instance type is available in one region, you cannot make the assumption that it will be available in all availability zones?

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The script that used for testing is:

aws ec2 describe-reserved-instances-offerings \
  --query "ReservedInstancesOfferings[?AvailabilityZone=='eu-central-1c'] [InstanceType]" \
  --output text --region eu-central-1 | grep c3.large

# c3.large not available on eu-central-1c

but

aws ec2 describe-reserved-instances-offerings \
  --query "ReservedInstancesOfferings[?AvailabilityZone=='eu-central-1a'] [InstanceType]" \
  --output text --region eu-central-1 | grep c3.large

# ... it is available on eu-central-1a

Answer 1

The availability of instance types can differ between availability zones in a region based on the documentation.

From the EC2 docs:

Some AWS resources might not be available in all regions and Availability Zones. Ensure that you can create the resources you need in the desired regions or Availability Zone before launching an instance in a specific Availability Zone.

Although resource is a vague term, an instance type should be considered a resource. So, it's possible that a certain instance type is not available in a certain zone but available in other availability zones. There are also cases where an instance of a certain type cannot be launched because AWS does not have enough capacity in that zone at the time of request.

Another important thing to note here is that the availability zones are not fixed for all accounts as described here.

To ensure that resources are distributed across the Availability Zones for a region, we independently map Availability Zones to identifiers for each account. For example, your Availability Zone us-east-1a might not be the same location as us-east-1a for another account. There's no way for you to coordinate Availability Zones between accounts.