Node Drain in KubeVirt


In a Kubernetes (k8s) cluster, the control plane(scheduler) is responsible for deploying workloads(pods, deployments, replicasets) on the worker nodes depending on the resource availability. What do we do with the workloads if the need arises for maintaining this node? Well, there is good news, node-drain feature and node maintenance operator(NMO) both come to our rescue in this situation.

This post discusses evicting the VMI(virtual machine images) and other resources from the node using node drain feature and NMO.


The environment used for writing this post is based on OpenShift 4 with 3 Masters and 3 Worker nodes.

  • HyperconvergedClusterOperator: The goal of the hyper-converged-cluster-operator (HCO) is to provide a single entry point for multiple operators (kubevirt, cdi, networking, etc) where users can deploy and configure them in a single object. This operator is sometimes referred to as a “meta operator” or an “operator for operators”. Most importantly, this operator doesn’t replace or interfere with OLM which is an open source toolkit to manage Kubernetes native applications, called Operators, in an effective, automated, and scalable way. Check for more information about OLM. It only creates operator CRs, which is the user’s prerogative.

In our cluster (3 master and 3 nodes) we’ll be able to see something similar to:

$oc get nodes   Ready  worker   14m   v1.13.4+27816e1b1    Ready  master   15m   v1.13.4+27816e1b1   Ready  worker   14m   v1.13.4+27816e1b1   Ready  master   14m   v1.13.4+27816e1b1   Ready  master   15m   v1.13.4+27816e1b1   Ready  worker   15m   v1.13.4+27816e1b1

To test the node eviction, there are two methods.

Method 1: Use kubectl node drain command

Before sending a node into maintenance state it is very much necessary to evict the resources on it, VMI’s, pods, deployments etc. One of the easiest option for us is to stick to the oc adm drain command. For this, select the node from the cluster from which you want the VMIs to be evicted

oc get nodes

Here, then issue the following command.

oc adm drain <node-name> --delete-local-data --ignore-daemonsets=true --force
  • --delete-local-data is used to remove any VMI’s that use emptyDir volumes, however the data in those volumes are ephemeral which means it is safe to delete after termination.

  • --ignore-daemonsets=true is a must needed flag because when VMI is deployed a daemon set named virt-handler will be running on each node. DaemonSets are not allowed to be evicted using kubectl drain. By default, if this command encounters a DaemonSet on the target node, the command will fail. This flag tells the command it is safe to proceed with the eviction and to just ignore DaemonSets.

  • flag tells the command to evict the pods that are managed by kubevirt

Evict a node

If you want to evict all pods from the node just use:

oc adm drain <node name> --delete-local-data --ignore-daemonsets=true --force

How to evacuate VMIs via Live Migration from a Node

If the LiveMigration feature gate is enabled, it is possible to specify an evictionStrategy on VMIs which will react with live-migrations on specific taints on nodes. The following snippet on a VMI ensures that the VMI is migrated if the taint is added to a node:

  evictionStrategy: LiveMigrate

Once the VMI is created, taint the node with

kubectl taint nodes foo

This command will then trigger a migration.

Behind the scenes a PodDisruptionBudget is created for each VMI which has an evictionStrategy defined. This ensures that evictions are be blocked on these VMIs and that we can guarantee that a VMI will be migrated instead of shut off.

Re-enabling a Node after Eviction

We have seen how to make the node unschedulable, now lets see how to re-enable the node.

The oc adm drain will result in the target node being marked as unschedulable. This means the node will not be eligible for running new VirtualMachineInstances or Pods.

If target node should become schedulable again, the following command must be run:

oc adm uncordon <node name>

Method 2: Use Node Maintenance Operator (NMO)

NMO is part of HyperConvergedClusterOperator, so we need to deploy it.

Either check:

Here will continue using the gist for demonstration purposes.

Observe the resources that get created after the HCO is installed

$oc get pods -n kubevirt-hyperconverged
NAME                                               READY   STATUS    RESTARTS   AGE
cdi-apiserver-769fcc7bdf-xgpt8                     1/1     Running   0          12m
cdi-deployment-8b64c5585-gq46b                     1/1     Running   0          12m
cdi-operator-77b8847b96-kx8rx                      1/1     Running   0          13m
cdi-uploadproxy-8dcdcbff-47lng                     1/1     Running   0          12m
cluster-network-addons-operator-584dff99b8-2c96w   1/1     Running   0          13m
hco-operator-59b559bd44-vpznq                      1/1     Running   0          13m
kubevirt-ssp-operator-67b78446f7-b9klr             1/1     Running   0          13m
kubevirt-web-ui-operator-9df6b67d9-f5l4l           1/1     Running   0          13m
node-maintenance-operator-6b464dc85-zd6nt          1/1     Running   0          13m
virt-api-7655b9696f-g48p8                          1/1     Running   1          12m
virt-api-7655b9696f-zfsw9                          1/1     Running   0          12m
virt-controller-7c4584f4bc-6lmxq                   1/1     Running   0          11m
virt-controller-7c4584f4bc-6m62t                   1/1     Running   0          11m
virt-handler-cfm5d                                 1/1     Running   0          11m
virt-handler-ff6c8                                 1/1     Running   0          11m
virt-handler-mcl7r                                 1/1     Running   1          11m
virt-operator-87d7c98b-fvvzt                       1/1     Running   0          13m
virt-operator-87d7c98b-xzc42                       1/1     Running   0          13m
virt-template-validator-76cbbd6f68-5fbzx           1/1     Running   0          12m

As seen from above HCO deploys the node-maintenance-operator.

Next, let’s install a kubevirt CR to start using VM workloads on worker nodes. Please feel free to follow the steps here and deploy a VMI as explained. Please feel free to check the video that explains the same

$oc get vms
testvm   2m13s   true

Deploy a node-maintenance-operator CR: As seen from above NMO is deployed from HCO, the purpose of this operator is to watch the node maintenance CustomResource(CR) called NodeMaintenance which mainly contains the node that needs a maintenance and the reason for it. The below actions are performed

  • If a NodeMaintenance CR is created: Marks the node as unschedulable, cordons it and evicts all the pods from that node
  • If a NodeMaintenance CR is deleted: Marks the node as schedulable, uncordons it, removes pod from maintenance.

To install the NMO, please follow upsream instructions at NMO

Either use HCO to create NMO Operator or deploy NMO operator as shown below

After you follow the instructions:

  1. Create a CRD
    oc create -f deploy/crds/nodemaintenance_crd.yaml created
  2. Create the NS
    oc create -f deploy/namespace.yaml
    namespace/node-maintenance-operator created
  3. Create a Service Account:
    oc create -f deploy/service_account.yaml
    serviceaccount/node-maintenance-operator created
  4. Create a ROLE
    oc create -f deploy/role.yaml created
  5. Create a ROLE Binding
    oc create -f deploy/role_binding.yaml created
  6. Then finally make sure to add the image version of the NMO operator in the deploy/operator.yml
  7. and then deploy the NMO Operator as shown
    oc create -f deploy/operator.yaml
    deployment.apps/node-maintenance-operator created

Finally, We can verify the deployment for the NMO Operator as below

oc get deployment -n node-maintenance-operator
NAME                        READY   UP-TO-DATE   AVAILABLE   AGE
node-maintenance-operator   1/1     1            1           4m23s

Now that the NMO operator is created, we can create the NMO CR which puts the node into maintenance mode (this CR has the info about the node->from which the pods needs to be evicted and the reason for the maintenance)

cat deploy/crds/nodemaintenance_cr.yaml

kind: NodeMaintenance
  name: nodemaintenance-xyz
  nodeName: <Node-Name>
  reason: "Test node maintenance"

For testing purpose, we can deploy a sample VM instance as shown:

kubectl apply -f

Now start the VM testvm

./virtctl start testvm

We can see that it’s up and running

kubectl get vmis
NAME     AGE   PHASE     IP            NODENAME
testvm   92s   Running

Also, we can see the status:

kubectl get vmis -o yaml testvm
  - ipAddress:
    mac: 0a:58:0a:83:00:11
    name: default
  migrationMethod: BlockMigration
  nodeName:    #NoteDown the nodeName
  phase: Running

Note down the node name and edit the nodemaintenance_cr.yaml file and then issue the CR manifest which sends the node into maintenance.

Now to evict the pods from the node, edit the node-maintenance_cr.yaml as shown:

cat deploy/crds/nodemaintenance_cr.yaml

kind: NodeMaintenance
  name: nodemaintenance-xyz
  reason: "Test node maintenance"

As soon as you apply the above CR, the current VM gets deployed in the other node,

oc apply -f deploy/crds/nodemaintenance_cr.yaml created

Which immediately evicts the VMI

kubectl get vmis
testvm   33s   Scheduling

kubectl get vmis
NAME     AGE    PHASE     IP            NODENAME
testvm   104s   Running   Ready,SchedulingDisabled   worker

When all of this happens, we can view the changes that are taking place with:

oc logs pods/node-maintenance-operator-645f757d5-89d6r -n node-maintenance-operator
{"level":"info","ts":1559681430.650298,"logger":"controller_nodemaintenance","msg":"Applying Maintenance mode on Node: with Reason: Test node maintenance","Request.Namespace":"","Request.Name":"nodemaintenance-xyz"}
{"level":"info","ts":1559681430.7509086,"logger":"controller_nodemaintenance","msg":"Taints: [{\"key\":\"\",\"effect\":\"NoSchedule\"},{\"key\":\"\",\"effect\":\"NoSchedule\"}] will be added to node"}
{"level":"info","ts":1559681430.7509348,"logger":"controller_nodemaintenance","msg":"Applying taint add on Node:"}
{"level":"info","ts":1559681430.7509415,"logger":"controller_nodemaintenance","msg":"Patchi{"level":"info","ts":1559681430.9903986,"logger":"controller_nodemaintenance","msg":"evicting pod \"virt-controller-b94d69456-b9dkw\"\n"}
{"level":"info","ts":1559681430.99049,"logger":"controller_nodemaintenance","msg":"evicting pod \"community-operators-5cb68db58-4m66j\"\n"}
{"level":"info","ts":1559681430.9905066,"logger":"controller_nodemaintenance","msg":"evicting pod \"alertmanager-main-1\"\n"}
{"level":"info","ts":1559681430.9905581,"logger":"controller_nodemaintenance","msg":"evicting pod \"virt-launcher-testvm-q5t7l\"\n"}
{"level":"info","ts":1559681430.9905746,"logger":"controller_nodemaintenance","msg":"evicting pod \"redhat-operators-6b6f6bd788-zx8nm\"\n"}
{"level":"info","ts":1559681430.990588,"logger":"controller_nodemaintenance","msg":"evicting pod \"image-registry-586d547bb5-t9lwr\"\n"}
{"level":"info","ts":1559681430.9906075,"logger":"controller_nodemaintenance","msg":"evicting pod \"kube-state-metrics-5bbd4c45d5-sbnbg\"\n"}
{"level":"info","ts":1559681430.9906383,"logger":"controller_nodemaintenance","msg":"evicting pod \"certified-operators-9f9f6fd5c-9ltn8\"\n"}
{"level":"info","ts":1559681430.9908028,"logger":"controller_nodemaintenance","msg":"evicting pod \"virt-api-59d7c4b595-dkpvs\"\n"}
{"level":"info","ts":1559681430.9906204,"logger":"controller_nodemaintenance","msg":"evicting pod \"router-default-6b57bcc884-frd57\"\n"}

Clearly we can see that the previous node went into SchedulingDisabled state and the VMI was evicted and placed into other node in the cluster. This demonstrates the node eviction using NMO.

VirtualMachine Evictions notes

The eviction of any VirtualMachineInstance that is owned by a VirtualMachine set to running=true will result in the VirtualMachineInstance being re-scheduled to another node.

The VirtualMachineInstance in this case will be forced to power down and restart on another node. In the future once KubeVirt introduces live migration support, the VM will be able to seamlessly migrate to another node during eviction.


The NMO achieved its aim of evicting the VMI’s successfully from the node, hence we can now safely repair/update the node and make it available for running the workloads again once the maintenance is over.