Period

If the metric isn't pushed for the specified period value, no data points are returned.

For example, if you enabled basic monitoring for an instance, EC2 pushes data points every five minutes. Let's say that EC2 pushes the data points at time stamps 12:00, 12:05, 12:10, and so on. Now, if you have your start time and end time within 12:01 and 12:04 and then try to retrieve the data points with a period of 60 seconds, you won't see any data points. It's a best practice to have your start time and end time extend beyond the minimum granularity offered by the metric (5 minutes in this case), or that you use a period that is greater than or equal to the minimum granularity that is offered by the metric.

Statistics

To retrieve percentile statistics for a metric, use ExtendedStatistic.

CloudWatch uses raw data points to calculate percentiles. If you publish data using a statistic set, you can retrieve percentile statistics for this data only if one of the following conditions are true:

• The SampleCount of the statistic set is 1.
• The Min and the Max of the statistic set are equal.

Unit

• If the specified unit is different from the one that is configured for the metric, no data points are returned.
• If you don't specify the unit argument, data points for all units are returned.

Start time and end time

• Format the start time and the end time arguments as specified in the GetMetricStatistics documentation.
• If no data points are pushed for the metric between the start time and end time, then no data points are returned.

Note: Data points with time stamps from 24 hours ago or longer can take at least 48 hours to become available for get-metric-statistics from the time they are submitted. For more information, see put-metric-data.