When you open the Monitoring tab in the EC2 console, you see CPU, network I/O, and disk I/O. But the metrics operators most want to see—memory usage and disk free space—are missing. The reason is simple: these metrics are only knowable from inside the guest OS, and the AWS hypervisor cannot see inside the guest OS. This is where the boundary of the shared responsibility model becomes apparent. CloudWatch Agent is software that runs inside that boundary—inside the guest OS—and exports metrics and logs to CloudWatch.
AWS EC2 runs on the Nitro hypervisor