The setting of systemReserved: ephemeral-storage in KubeletConfig is not working as expected. 

4.10.z, may exist on other OCP versions as well. 

always

1. Create a KubeletConfig on the node:

apiVersion: machineconfiguration.openshift.io/v1
kind: KubeletConfig
metadata:
  name: system-reserved-config
spec:
  machineConfigPoolSelector:
    matchLabels:
      pools.operator.machineconfiguration.openshift.io/master: ""
  kubeletConfig:
    systemReserved:
      cpu: 500m
      memory: 500Mi
      ephemeral-storage: 10Gi


2. Check node allocatable storage with command: oc describe node |grep -C 5 ephemeral-storage

The Allocatable:ephemeral-storage on the node is not capacity.ephemeral-storage - systemReserved.ephemeral-storage - eviction-thresholds (10% of the capacity.ephemeral-storage by default)  

The Allocatable:ephemeral-storage on the node should be capacity.ephemeral-storage - systemReserved.ephemeral-storage - eviction-thresholds (10% of the capacity.ephemeral-storage by default) 

The root cause might be: process argument '--system-reserved=cpu=500m,memory=500Mi' overwrote the setting in /etc/kubernetes/kubelet.conf, one example:

root        6824       1 27 Sep30 ?        1-09:00:24 kubelet --config=/etc/kubernetes/kubelet.conf --bootstrap-kubeconfig=/etc/kubernetes/kubeconfig --kubeconfig=/var/lib/kubelet/kubeconfig --container-runtime=remote --container-runtime-endpoint=/var/run/crio/crio.sock --runtime-cgroups=/system.slice/crio.service --node-labels=node-role.kubernetes.io/master,node.openshift.io/os_id=rhcos --node-ip=192.168.58.47 --minimum-container-ttl-duration=6m0s --cloud-provider= --volume-plugin-dir=/etc/kubernetes/kubelet-plugins/volume/exec --hostname-override= --register-with-taints=node-role.kubernetes.io/master=:NoSchedule --pod-infra-container-image=quay.io/openshift-release-dev/ocp-v4.0-art-dev@sha256:4a7b6408460148cb73c59677dbc2c261076bc07226c43b0c9192cc70aef5ba62 --system-reserved=cpu=500m,memory=500Mi --v=2 --housekeeping-interval=30s