Put nodes into maintenance mode

When you need to repair or maintain nodes, you should first put the nodes into maintenance mode. Putting nodes into maintenance mode safely drains their pods/workloads and excludes the nodes from pod scheduling. In maintenance mode, you can work on your nodes without a risk of disrupting pod traffic.

How it works

Anthos clusters on bare metal provides a way to place nodes into maintenance mode. This approach lets other cluster components correctly know that the node is in maintenance mode. When you place a node in maintenance mode, no additional pods can be scheduled on the node, and existing pods are stopped.

Instead of using maintenance mode, you can manually use Kubernetes commands such as kubectl cordon and kubectl drain on a specific node. If you run Anthos clusters on bare metal version 1.12.0 (anthosBareMetalVersion: 1.12.0) or lower, see the known issue on Nodes uncordoned if you don't use the maintenance mode procedure.

When you use the maintenance mode process, Anthos clusters on bare metal does the following:

Node taints are added to specified nodes to indicate that no pods can be scheduled or executed on the nodes.
A 20-minute timeout is enforced to ensure nodes don't get stuck waiting for pods to stop. Pods might not stop if they are configured to tolerate all taints or they have finalizers. Anthos clusters on bare metal attempts to stop all pods, but if the timeout is exceeded, the node is put into maintenance mode. This timeout prevents running pods from blocking upgrades.
If you have a VM-based workload running on the node, Anthos clusters on bare metal applies a NodeSelector to the virtual machine instance (VMI) Pod, then stops the Pod. The NodeSelector ensures that the VMI Pod is restarted on the same node when the node is removed from maintenance mode.

Put a node into maintenance mode

Choose the nodes you want to put into maintenance mode by specifying IP ranges for the selected nodes under maintenanceBlocks in your cluster configuration file. The nodes you choose must be in a ready state, and functioning in the cluster.

To put nodes into maintenance mode:

Edit the cluster configuration file to select the nodes you want to put into maintenance mode.

You can edit the configuration file with an editor of your choice, or you can edit the cluster custom resource directly by running the following command:
```
kubectl -n CLUSTER_NAMESPACE edit cluster CLUSTER_NAME
```
Replace the following:
- CLUSTER_NAMESPACE: the namespace of the cluster.
- CLUSTER_NAME: the name of the cluster.
Add the maintenanceBlocks section to the cluster configuration file to specify either a single IP address, or an address range, for nodes you want to put into maintenance mode.

The following sample shows how to select multiple nodes by specifying a range of IP addresses:
```
metadata:
  name: my-cluster
  namespace: cluster-my-cluster
spec:
  maintenanceBlocks:
    cidrBlocks:
    - 172.16.128.1-172.16.128.64
```
Save and apply the updated cluster configuration.

Anthos clusters on bare metal starts putting the nodes into maintenance mode.

Run the following command to get the status of the nodes in your cluster:

kubectl get nodes -n CLUSTER_NAME

The response is something like the following:

NAME                              STATUS                     ROLES    AGE     VERSION
user-anthos-baremetal-01          Ready                      master   2d22h   v1.17.8-gke.16
user-anthos-baremetal-04          Ready                      &lt;none&gt;   2d22h   v1.17.8-gke.16
user-anthos-baremetal-05          Ready,SchedulingDisabled   &lt;none&gt;   2d22h   v1.17.8-gke.16
user-anthos-baremetal-06          Ready                      &lt;none&gt;   2d22h   v1.17.8-gke.16

A status of SchedulingDisabled indicates that a node is in maintenance mode.

Run the following command to get the number of nodes in maintenance mode:
```
kubectl get nodepools
```
The response should look something like the following output:
```
NAME   READY   RECONCILING   STALLED   UNDERMAINTENANCE   UNKNOWN
np1    3       0             0         1                  0
```
This UNDERMAINTENANCE column in this sample shows that one node is in maintenance mode.

Anthos clusters on bare metal also adds the following taints to nodes when they are put into maintenance mode:
- baremetal.cluster.gke.io/maintenance:NoExecute
- baremetal.cluster.gke.io/maintenance:NoSchedule

Remove a node from maintenance mode

To remove nodes from maintenance mode:

Edit the cluster configuration file to clear the nodes you want to remove from maintenance mode.

You can edit the configuration file with an editor of your choice, or you can edit the cluster custom resource directly by running the following command:
```
kubectl -n CLUSTER_NAMESPACE edit cluster CLUSTER_NAME
```
Replace the following:
- CLUSTER_NAMESPACE: the namespace of the cluster.
- CLUSTER_NAME: the name of the cluster.
Either edit the IP addresses to remove specific nodes from maintenance mode or remove the maintenanceBlocks section remove all does from maintenance mode.
Save and apply the updated cluster configuration.
Use kubectl commands to check the status of your nodes.