Let's say you just upgraded your AOS to the latest version and now you are receiving this alert in your environment.
"Disk space usage for root on Controller VM has exceeded 80%"
The Nutanix CVM is what runs the Nutanix software and serves all of the I/O operations for the hypervisor and all VMs running on that host.
Not sure how to bring the root partition on the CVM below 80% and which document to consult to?
The following knowledgebase articles might help you in resolving the alert.