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AWS Local Zone tasks

After installing OpenShift Container Platform on Amazon Web Services (AWS), you can further configure AWS Local Zones and an edge compute pool, so that you can expand and customize your cluster to meet your needs.

Extend existing clusters to use AWS Local Zones

As a postinstallation task, you can extend an existing OpenShift Container Platform cluster on Amazon Web Services (AWS) to use AWS Local Zones.

Extending nodes to Local Zone locations comprises the following steps:

  • Adjusting the cluster-network maximum transmission unit (MTU)

  • Opting in the Local Zone group to AWS Local Zones

  • Creating a subnet in the existing VPC for a Local Zone location

  • Creating the machine set manifest, and then creating a node in each Local Zone location

Before you extend an existing OpenShift Container Platform cluster on AWS to use Local Zones, check that the existing VPC contains available Classless Inter-Domain Routing (CIDR) blocks. These blocks are needed for creating the subnets.
Additional resources

  • For more information about AWS Local Zones, the supported instances types, and services, see AWS Local Zones features in the AWS documentation.

Edge compute pools and AWS Local Zones

Edge worker nodes are tainted worker nodes that run in AWS Local Zones locations.

When deploying a cluster that uses Local Zones, consider the following points:

  • Amazon EC2 instances in the Local Zones are more expensive than Amazon EC2 instances in the Availability Zones.

  • Latency between applications and end users is lower in Local Zones, and latency might vary by location. A latency impact exists for some workloads if, for example, ingress traffic is mixed between Local Zones and Availability Zones.

Generally, the maximum transmission unit (MTU) between an Amazon EC2 instance in a Local Zone and an Amazon EC2 instance in the Region is 1300. For more information, see How Local Zones work in the AWS documentation. The cluster network MTU must be always less than the EC2 MTU to account for the overhead. The specific overhead is determined by the network plugin, for example:

  • OVN-Kubernetes: 100 bytes

  • OpenShift SDN: 50 bytes

The network plugin can provide additional features, like IPsec, that also must be decreased the MTU. For additional information, see the documentation.

OpenShift Container Platform 4.12 introduced a new compute pool, edge, that is designed for use in remote zones. The edge compute pool configuration is common between AWS Local Zones locations. Because of the type and size limitations of resources like EC2 and EBS on Local Zone resources, the default instance type can vary from the traditional worker pool.

The default Elastic Block Store (EBS) for Local Zone locations is gp2, which differs from the regular worker pool. The instance type used for each Local Zone on edge compute pool also might differ from worker pools, depending on the instance offerings on the zone.

The edge compute pool creates new labels that developers can use to deploy applications onto AWS Local Zones nodes. The new labels are:

  • node-role.kubernetes.io/edge=''

  • machine.openshift.io/zone-type=local-zone

  • machine.openshift.io/zone-group=$ZONE_GROUP_NAME

By default, the machine sets for the edge compute pool defines the taint of NoSchedule to prevent regular workloads from spreading on Local Zone instances. Users can only run user workloads if they define tolerations in the pod specification.

Changing the Cluster Network MTU to support AWS Local Zones subnets

You might need to change the maximum transmission unit (MTU) value for the cluster network so that your cluster infrastructure can support Local Zone subnets.

About the cluster MTU

During installation the maximum transmission unit (MTU) for the cluster network is detected automatically based on the MTU of the primary network interface of nodes in the cluster. You do not usually need to override the detected MTU.

You might want to change the MTU of the cluster network for several reasons:

  • The MTU detected during cluster installation is not correct for your infrastructure.

  • Your cluster infrastructure now requires a different MTU, such as from the addition of nodes that need a different MTU for optimal performance.

You can change the cluster MTU for only the OVN-Kubernetes and OpenShift SDN cluster network plugins.

Service interruption considerations

When you initiate an MTU change on your cluster the following effects might impact service availability:

  • At least two rolling reboots are required to complete the migration to a new MTU. During this time, some nodes are not available as they restart.

  • Specific applications deployed to the cluster with shorter timeout intervals than the absolute TCP timeout interval might experience disruption during the MTU change.

MTU value selection

When planning your MTU migration there are two related but distinct MTU values to consider.

  • Hardware MTU: This MTU value is set based on the specifics of your network infrastructure.

  • Cluster network MTU: This MTU value is always less than your hardware MTU to account for the cluster network overlay overhead. The specific overhead is determined by your network plugin:

    • OVN-Kubernetes: 100 bytes

    • OpenShift SDN: 50 bytes

If your cluster requires different MTU values for different nodes, you must subtract the overhead value for your network plugin from the lowest MTU value that is used by any node in your cluster. For example, if some nodes in your cluster have an MTU of 9001, and some have an MTU of 1500, you must set this value to 1400.

To avoid selecting an MTU value that is not acceptable by a node, verify the maximum MTU value (maxmtu) that is accepted by the network interface by using the ip -d link command.
How the migration process works

The following table summarizes the migration process by segmenting between the user-initiated steps in the process and the actions that the migration performs in response.

Table 1. Live migration of the cluster MTU
User-initiated steps OpenShift Container Platform activity

Set the following values in the Cluster Network Operator configuration:

  • spec.migration.mtu.machine.to

  • spec.migration.mtu.network.from

  • spec.migration.mtu.network.to

Cluster Network Operator (CNO): Confirms that each field is set to a valid value.

  • The mtu.machine.to must be set to either the new hardware MTU or to the current hardware MTU if the MTU for the hardware is not changing. This value is transient and is used as part of the migration process. Separately, if you specify a hardware MTU that is different from your existing hardware MTU value, you must manually configure the MTU to persist by other means, such as with a machine config, DHCP setting, or a Linux kernel command line.

  • The mtu.network.from field must equal the network.status.clusterNetworkMTU field, which is the current MTU of the cluster network.

  • The mtu.network.to field must be set to the target cluster network MTU and must be lower than the hardware MTU to allow for the overlay overhead of the network plugin. For OVN-Kubernetes, the overhead is 100 bytes and for OpenShift SDN the overhead is 50 bytes.

If the values provided are valid, the CNO writes out a new temporary configuration with the MTU for the cluster network set to the value of the mtu.network.to field.

Machine Config Operator (MCO): Performs a rolling reboot of each node in the cluster.

Reconfigure the MTU of the primary network interface for the nodes on the cluster. You can use a variety of methods to accomplish this, including:

  • Deploying a new NetworkManager connection profile with the MTU change

  • Changing the MTU through a DHCP server setting

  • Changing the MTU through boot parameters

N/A

Set the mtu value in the CNO configuration for the network plugin and set spec.migration to null.

Machine Config Operator (MCO): Performs a rolling reboot of each node in the cluster with the new MTU configuration.

Changing the cluster MTU

As a cluster administrator, you can change the maximum transmission unit (MTU) for your cluster. The migration is disruptive and nodes in your cluster might be temporarily unavailable as the MTU update rolls out.

Prerequisites

  • You installed the OpenShift CLI (oc).

  • You are logged in to the cluster with a user with cluster-admin privileges.

  • You identified the target MTU for your cluster. The correct MTU varies depending on the network plugin that your cluster uses:

    • OVN-Kubernetes: The cluster MTU must be set to 100 less than the lowest hardware MTU value in your cluster.

    • OpenShift SDN: The cluster MTU must be set to 50 less than the lowest hardware MTU value in your cluster.

Procedure

To increase or decrease the MTU for the cluster network complete the following procedure.

  1. To obtain the current MTU for the cluster network, enter the following command:

    $ oc describe network.config cluster
    Example output
    ...
Status:
  Cluster Network:
    Cidr:               10.217.0.0/22
    Host Prefix:        23
  Cluster Network MTU:  1400
  Network Type:         OpenShiftSDN
  Service Network:
    10.217.4.0/23
...

  2. To begin the MTU migration, specify the migration configuration by entering the following command. The Machine Config Operator performs a rolling reboot of the nodes in the cluster in preparation for the MTU change.

    $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": { "mtu": { "network": { "from": <overlay_from>, "to": <overlay_to> } , "machine": { "to" : <machine_to> } } } } }'

    where:

    <overlay_from>

    Specifies the current cluster network MTU value.

    <overlay_to>

    Specifies the target MTU for the cluster network. This value is set relative to the value for <machine_to> and for OVN-Kubernetes must be 100 less and for OpenShift SDN must be 50 less.

    <machine_to>

    Specifies the MTU for the primary network interface on the underlying host network.

    Example that increases the cluster MTU
    $ oc patch Network.operator.openshift.io cluster --type=merge --patch \
  '{"spec": { "migration": { "mtu": { "network": { "from": 1400, "to": 9000 } , "machine": { "to" : 9100} } } } }'

  3. As the MCO updates machines in each machine config pool, it reboots each node one by one. You must wait until all the nodes are updated. Check the machine config pool status by entering the following command:

    $ oc get mcp

    A successfully updated node has the following status: UPDATED=true, UPDATING=false, DEGRADED=false.

    By default, the MCO updates one machine per pool at a time, causing the total time the migration takes to increase with the size of the cluster.

  4. Confirm the status of the new machine configuration on the hosts:

    1. To list the machine configuration state and the name of the applied machine configuration, enter the following command:

      $ oc describe node | egrep "hostname|machineconfig"
      Example output
      kubernetes.io/hostname=master-0
machineconfiguration.openshift.io/currentConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/desiredConfig: rendered-master-c53e221d9d24e1c8bb6ee89dd3d8ad7b
machineconfiguration.openshift.io/reason:
machineconfiguration.openshift.io/state: Done

      Verify that the following statements are true:

      • The value of machineconfiguration.openshift.io/state field is Done.

      • The value of the machineconfiguration.openshift.io/currentConfig field is equal to the value of the machineconfiguration.openshift.io/desiredConfig field.

    2. To confirm that the machine config is correct, enter the following command:

      $ oc get machineconfig <config_name> -o yaml | grep ExecStart

      where <config_name> is the name of the machine config from the machineconfiguration.openshift.io/currentConfig field.

      The machine config must include the following update to the systemd configuration:

      ExecStart=/usr/local/bin/mtu-migration.sh