EC2 System and Instance Checks
In Cloudwatch, you have System and Instance checks. One checks the health of the physical host your EC2 instance is running on, the other checks the health of the EC2 instance itself:
Last night we had an instance that failed the instance checks, and we were alerted by Cloudwatch itself. We did get an alert that the agent was disconnected, but no instance check failures. Are these metrics being monitored? They really are the base of EC2 monitoring.