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I have a CPUUtilization alarm setup for my ec2 instance. The settings for the alarm are:

CPUUtilization >= 95% for 15 consecutive periods of 1 minute (15mins total)

We are still receiving alert emails even though the instance does not appear to enter the defined alarm state.

The email alarm my team and I are receiving explains:

You are receiving this email because your Amazon CloudWatch Alarm ... has entered the ALARM state, because "Threshold Crossed: 3 datapoints were greater than or equal to the threshold (95.0). The most recent datapoints: [99.466, 98.45]...

3 datapoints? It is my understanding that my setting of 15 consecutive periods of 1 minute should only alert if 15 datapoints were greater than 95%.

Am I misunderstanding the text of the email? Are my alarm settings conflicting with something?

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Your EC2 instance must be enabled for monitoring at 1-minute intervals. If you have not enabled detailed monitoring on your EC2 instance, then you would be collecting data in 5-minute intervals. 3 consecutive periods of 5-minute intervals would be 15 minutes.

I'm not certain, but after reviewing some of my own Cloudwatch alarms and playing with a new one in the console... it seems like, in this case, the alarm state triggers based on minutes instead of periods -- we just define minutes in terms of periods at alarm creation time. This seems sensible to me -- otherwise your alarm wouldn't ever be able to enter the alarm state if detailed (1-minute) monitoring was disabled.

Regarding detailed monitoring: I would turn it on for this case, if it is disabled. If you are using basic (5-minute) monitoring, the 3 data points don't necessarily mean that CPU Utilization has been >= 95% for 15 consecutive minutes. It rather means that CPU Utilization was >= 95% at the time the data was sampled, for 3 consecutive samplings.

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Amazon let you create alarms for 1 minute period even if you do not have detailed monitoring enabled but there is no data for 4 minutes. If you see the CloudWatch console, your alarm should be in Insufficient state for 4 minutes and change into OK every 5th minute. So you will be all good if you just change period to 5 minutes or maybe enable detailed monitoring.

CloudWatch Periods:

It depends on what type of period you select, not on its value.

If you select minutes then 1 consecutive period is 1 minute and 10 consecutive periods are 10 minutes.

If you select hours then 1 consecutive period is 1 hour and 10 consecutive periods are 10 hours.

If you select days then 1 consecutive period is 1 day and 10 consecutive periods are 10 days.

In your case, you selected period as 15 minutes and then 15 consecutive periods but there was data available only every 5th minutes therefore it has only 3 data points.

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