Hi, CloudWatch can ingest metrics with a resolution of 1 second, but the ability to leverage that resolution is not currently offered by detailed monitoring, so only custom metrics users take advantage of it.
Monitoring type is not just about the resolution: Basic monitoring includes some 1-min metrics at no charge (e.g. StatusCheck on EC2- https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/viewing_metrics_with_cloudwatch.html). And when you turn on detailed monitoring, in addition to getting 1-minute resolution for many metrics, you also get additional metrics. For example on EC2: "For the instances where you've enabled detailed monitoring, you can also get aggregated data across groups of similar instances." (cf. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-cloudwatch-new.html)
Regarding sub-minute resolution: If using custom metrics, you can ingest your observations with sub-minute resolution, down to 1-second resolution, then plot them at 1-second resolution, or use CloudWatch statistics to surface means or percentiles over different time resolutions (e.g. 5/10/30 seconds), or alarm on them at sub-minute (e.g. 10-second) frequency.
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