Node maintenance - Nodes | Virtualization

Eviction strategies
- Configuring a VM eviction strategy using the command line
- Configuring a cluster eviction strategy by using the command line
Run strategies
- Run strategies
- Configuring a VM run strategy by using the command line
Maintaining bare metal nodes
Additional resources

Nodes can be placed into maintenance mode by using the oc adm utility or NodeMaintenance custom resources (CRs).

The node-maintenance-operator (NMO) is no longer shipped with OKD Virtualization. It is deployed as a standalone Operator from the OperatorHub in the OKD web console or by using the OpenShift CLI (oc).

For more information on remediation, fencing, and maintaining nodes, see the Workload Availability for Red Hat OpenShift documentation.

Virtual machines (VMs) must have a persistent volume claim (PVC) with a shared ReadWriteMany (RWX) access mode to be live migrated.

The Node Maintenance Operator watches for new or deleted NodeMaintenance CRs. When a new NodeMaintenance CR is detected, no new workloads are scheduled and the node is cordoned off from the rest of the cluster. All pods that can be evicted are evicted from the node. When a NodeMaintenance CR is deleted, the node that is referenced in the CR is made available for new workloads.

Using a NodeMaintenance CR for node maintenance tasks achieves the same results as the oc adm cordon and oc adm drain commands using standard OKD custom resource processing.

Eviction strategies

Placing a node into maintenance marks the node as unschedulable and drains all the VMs and pods from it.

You can configure eviction strategies for virtual machines (VMs) or for the cluster.

VM eviction strategy

The VM LiveMigrate eviction strategy ensures that a virtual machine instance (VMI) is not interrupted if the node is placed into maintenance or drained. VMIs with this eviction strategy will be live migrated to another node.

You can configure eviction strategies for virtual machines (VMs) by using the web console or the command line.

The default eviction strategy is LiveMigrate. A non-migratable VM with a LiveMigrate eviction strategy might prevent nodes from draining or block an infrastructure upgrade because the VM is not evicted from the node. This situation causes a migration to remain in a Pending or Scheduling state unless you shut down the VM manually.

You must set the eviction strategy of non-migratable VMs to LiveMigrateIfPossible, which does not block an upgrade, or to None, for VMs that should not be migrated.

Cluster eviction strategy: You can configure an eviction strategy for the cluster to prioritize workload continuity or infrastructure upgrade.

Configuring a cluster eviction strategy is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.

Table 1. Cluster eviction strategies
Eviction strategy	Description	Interrupts workflow	Blocks upgrades
`LiveMigrate` ¹	Prioritizes workload continuity over upgrades.	No	Yes ²
`LiveMigrateIfPossible`	Prioritizes upgrades over workload continuity to ensure that the environment is updated.	Yes	No
`None` ³	Shuts down VMs with no eviction strategy.	Yes	No

Default eviction strategy for multi-node clusters.
If a VM blocks an upgrade, you must shut down the VM manually.
Default eviction strategy for single-node OpenShift.

Configuring a VM eviction strategy using the command line

You can configure an eviction strategy for a virtual machine (VM) by using the command line.

You must set the eviction strategy of non-migratable VMs to LiveMigrateIfPossible, which does not block an upgrade, or to None, for VMs that should not be migrated.

Procedure

Edit the VirtualMachine resource by running the following command:

$ oc edit vm <vm_name> -n <namespace>

Example eviction strategy

apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  name: <vm_name>
spec:
  template:
    spec:
      evictionStrategy: LiveMigrateIfPossible (1)
# ...

1	Specify the eviction strategy. The default value is `LiveMigrate`.

Restart the VM to apply the changes:

$ virtctl restart <vm_name> -n <namespace>

Configuring a cluster eviction strategy by using the command line

You can configure an eviction strategy for a cluster by using the command line.

For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.

Procedure

Edit the hyperconverged resource by running the following command:

$ oc edit hyperconverged kubevirt-hyperconverged -n kubevirt-hyperconverged

Set the cluster eviction strategy as shown in the following example:

Example cluster eviction strategy

apiVersion: hco.kubevirt.io/v1beta1
kind: HyperConverged
metadata:
  name: kubevirt-hyperconverged
spec:
  evictionStrategy: LiveMigrate
# ...

Run strategies

A virtual machine (VM) configured with spec.running: true is immediately restarted. The spec.runStrategy key provides greater flexibility for determining how a VM behaves under certain conditions.

The spec.runStrategy and spec.running keys are mutually exclusive. Only one of them can be used.

A VM configuration with both keys is invalid.

Run strategies

The spec.runStrategy key has four possible values:

Always: The virtual machine instance (VMI) is always present when a virtual machine (VM) is created on another node. A new VMI is created if the original stops for any reason. This is the same behavior as running: true.
RerunOnFailure: The VMI is re-created on another node if the previous instance fails. The instance is not re-created if the VM stops successfully, such as when it is shut down.
Manual: You control the VMI state manually with the start, stop, and restart virtctl client commands. The VM is not automatically restarted.
Halted: No VMI is present when a VM is created. This is the same behavior as running: false.

Different combinations of the virtctl start, stop and restart commands affect the run strategy.

The following table describes a VM’s transition between states. The first column shows the VM’s initial run strategy. The remaining columns show a virtctl command and the new run strategy after that command is run.

Table 2. Run strategy before and after `virtctl` commands
Initial run strategy	Start	Stop	Restart
Always	-	Halted	Always
RerunOnFailure	-	Halted	RerunOnFailure
Manual	Manual	Manual	Manual
Halted	Always	-	-

If a node in a cluster installed by using installer-provisioned infrastructure fails the machine health check and is unavailable, VMs with runStrategy: Always or runStrategy: RerunOnFailure are rescheduled on a new node.

Configuring a VM run strategy by using the command line

You can configure a run strategy for a virtual machine (VM) by using the command line.

The spec.runStrategy and spec.running keys are mutually exclusive. A VM configuration that contains values for both keys is invalid.

Procedure

Edit the VirtualMachine resource by running the following command:

$ oc edit vm <vm_name> -n <namespace>

Example run strategy

apiVersion: kubevirt.io/v1
kind: VirtualMachine
spec:
  runStrategy: Always
# ...

Maintaining bare metal nodes

When you deploy OKD on bare metal infrastructure, there are additional considerations that must be taken into account compared to deploying on cloud infrastructure. Unlike in cloud environments where the cluster nodes are considered ephemeral, re-provisioning a bare metal node requires significantly more time and effort for maintenance tasks.

When a bare metal node fails, for example, if a fatal kernel error happens or a NIC card hardware failure occurs, workloads on the failed node need to be restarted elsewhere else on the cluster while the problem node is repaired or replaced. Node maintenance mode allows cluster administrators to gracefully power down nodes, moving workloads to other parts of the cluster and ensuring workloads do not get interrupted. Detailed progress and node status details are provided during maintenance.

Additional resources

About live migration