We've found that CloudWatch is useful for some system and I/O related metrics (i.e., the from the EBS SAN's point of view) but less useful for others (i.e., metrics from the VM's point of view). It's worthwhile to install a monitoring agent on the VM that can collect I/O latency and other statistics there too. We use Datadog, but there are lots of options out there (collectd etc.).

This is a topic that we're continuing to iterate on between EBS, CloudWatch, and the AWS console team for metrics. When a newly introduced behavior makes step function changes in graphs displayed in the AWS console it doesn't meet the principle of least astonishment.

We're also continuing to investigate the reported latency in the console. Because this is a derived metric from VolumeTotal{Read,Write}Time and Volume{Read,Write}Ops, there may be a miscalculation happening due to the change in dimensions.