Experiencing InsufficientInstanceCapacity despite using Capacity Reservation

Question

Hi folks,

I have a capacity reservation with 1 instance available for use:

```
$ aws ec2 describe-capacity-reservations --capacity-reservation-ids cr-00d78c6805551161a
{
    "CapacityReservations": [
        {
            "CapacityReservationId": "cr-00d78c6805551161a",
...
            "InstanceType": "c5n.9xlarge",
            "InstancePlatform": "Linux/UNIX",
            "AvailabilityZone": "us-east-1d",
...
            "AvailableInstanceCount": 1,
...
}

```

I also have a stopped instance with matching instance type/platform/AZ assigned to this capacity:

```

$ aws ec2 describe-instances --instance-ids $myinstance
{
    "Reservations": [
        {
            "Groups": [],
            "Instances": [
                {
                    "AmiLaunchIndex": 0,
                    "ImageId": "ami-6ed35e06",
                    "InstanceId": "i-2d1ff5cc",
                    "InstanceType": "c5n.9xlarge",
....
 "Placement": {
                         "AvailabilityZone": "us-east-1d",
....
                    },
                    "CapacityReservationSpecification": {
                        "CapacityReservationTarget": {
                            "CapacityReservationId": "cr-00d78c6805551161a"
                        }
                    },
....
                  "PlatformDetails": "Linux/UNIX",
....
}

```

However I cannot start it:

```

$ aws ec2 start-instances --instance-ids $myinstance

An error occurred (InsufficientInstanceCapacity) when calling the StartInstances operation (reached max retries: 2): Insufficient capacity.
```

I must be confused as I would expect that the capacity reservation would be used.

Why does the instance fail to start?

I asked AWS support 2 days ago, but haven't made progress yet.

Answer

Attaching the document link that confirms the finding about Placement Groups and On-Demand Capacity Reservations:
- https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-reservations.html#capacity-reservations-limitations

Which says, Capacity Reservations can't be created in placement groups.

---

Thank you for confirming the response you received from the internal team. Through AWS Support cases, AWS engineers are able to check instance configurations at a much more granular level.

Experiencing InsufficientInstanceCapacity despite using Capacity Reservation

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