Received "ALARM: "FreeSpace90GBAlarm" in US East (Ohio)" but can't tell which RDS instance

This response was not generated by gen AI. Ha ha.

I have these comments:

1. To find the metric in the RDS console:

Navigate to your instance.
Select Monitoring
Check that "Monitoring" is selected in the dropdown.
You could search for FreeStorageSpace
You should see a graph of FreeStorageSpace

2. It looks like you have set up the alarm from the CloudWatch console. However, the alarm e-mail says:

- Dimensions:    (none)

The alarm will not work unless Dimensions is set. The dimension of each alarm should refer to one DB instance. Try this:

CloudWatch
All alarms
Create alarm
Select metric
Enter FreeStorageSpace
You should see multiple choices, such as:
RDS > DBInstanceIdentifier
RDS > Across All Databases
RDS > DatabaseClass
RDS > EngineName
TrustedAdvisor > Check Metrics

Click on RDS > DBInstanceIdentifier
Select the DBInstanceIdentifier that you want
Click SelectMetric
Click Lower
Enter 96636764160
Click Next
Select your notification
Click Next
Enter a name
Click Next
Press Create Alarm

3. Some metrics are absolute values, not percentages. In that case, the threshold would need to be different for each DB instance. It is impractical to configure alarms for a high number of DB Instances on the console because it is tedious and error-prone. Also, the approach does not account for new instances. You could automate the configuration with the python boto3 library. You could get the list of db instances and the sizes of each by calling rds describe_db_instances.

You could make a call to cloudwatch such as:

put_metric_alarm(
                AlarmName='FreeSpace90GBAlarm',
                AlarmDescription='FreeSpace90GB Alarm test to dc email',
                MetricName='FreeStorageSpace',
                Namespace='AWS/RDS',
                Dimensions=[
                    {
                        'Name': 'DBInstanceIdentifier',
                        'Value': db_instance_identifier
                    }
                ],
                Statistic='Minimum',
                Period=60,
                EvaluationPeriods=2,
                Threshold=threshold,
                ComparisonOperator='LessThanThreshold',
                AlarmActions=[
                    'arn:aws:sns:us-east-2:910286192445:some-topic'
                ]
            )

where db_instance_identifier comes from the loop and threshold you compute based on db instance AllocatedStorage.

I implemented this a few years back, but deleted that code on complexity grounds.

4. RDS has percent-based events instead. You can subscribe to them on the RDS console. The RDS event thresholds are not configurable. The storage threshold starts at 10% remaining. You get an alert at each 1% step down to 0%.