YARN considers any node with disk utilization exceeding the value specified under the property yarn.nodemanager.disk-health-checker.max-disk-utilization-per-disk-percentage (in yarn-site.xml) to be unhealthy. Ample disk space is critical to ensure uninterrupted operation of a Hadoop cluster, and large numbers of unhealthyNodes (the number to alert on depends on the size of your cluster) should be quickly investigated and resolved.