Understanding OKD update duration

OKD update duration varies based on the deployment topology. This page helps you understand the factors that affect update duration and estimate how long the cluster update takes in your environment.

Prerequisites

• You are familiar with OpenShift Container Platform architecture and OpenShift Container Platform updates.

Factors affecting update duration

The following factors can affect your cluster update duration:

• The reboot of compute nodes to the new machine configuration by Machine Config Operator (MCO)

  • The value of MaxUnavailable in the machine config pool

  • The minimum number or percentages of replicas set in pod disruption budget (PDB)

• The number of nodes in the cluster

• The health of the cluster nodes

Cluster update phases

In OKD, the cluster update happens in two phases:

• Cluster Version Operator (CVO) target update payload deployment

• Machine Config Operator (MCO) node updates

Cluster Version Operator target update payload deployment

The Cluster Version Operator (CVO) retrieves the target update release image and applies to the cluster. All components which run as pods are updated during this phase, whereas the host components are updated by the Machine Config Operator (MCO). This process might take 60 to 120 minutes.

Additional resources

• Cluster Version Operator (CVO) overview

Machine Config Operator node updates

The Machine Config Operator (MCO) applies a new machine configuration to each control plane and compute node. During this process, the MCO performs the following sequential actions on each node of the cluster:

1. Cordon and drain all the nodes

2. Update the operating system (OS)

3. Reboot the nodes

4. Uncordon all nodes and schedule workloads on the node

The time to complete this process depends on several factors including the node and infrastructure configuration. This process might take 5 or more minutes to complete per node.

In addition to MCO, you should consider the impact of the following parameters:

• The control plane node update duration is predictable and oftentimes shorter than compute nodes, because the control plane workloads are tuned for graceful updates and quick drains.

• You can update the compute nodes in parallel by setting the maxUnavailable field to greater than 1 in the Machine Config Pool (MCP). The MCO cordons the number of nodes specified in maxUnavailable and marks them unavailable for update.

• When you increase maxUnavailable on the MCP, it can help the pool to update more quickly. However, if maxUnavailable is set too high, and several nodes are cordoned simultaneously, the pod disruption budget (PDB) guarded workloads could fail to drain because a schedulable node cannot be found to run the replicas. If you increase maxUnavailable for the MCP, ensure that you still have sufficient schedulable nodes to allow PDB guarded workloads to drain.

• Before you begin the update, you must ensure that all the nodes are available. Any unavailable nodes can significantly impact the update duration because the node unavailability affects the maxUnavailable and pod disruption budgets.

  To check the status of nodes from the terminal, run the following command:

  $ oc get node

  Example Output

  NAME                                        STATUS                      ROLES   AGE     VERSION
  ip-10-0-137-31.us-east-2.compute.internal   Ready,SchedulingDisabled    worker  12d     v1.23.5+3afdacb
  ip-10-0-151-208.us-east-2.compute.internal  Ready                       master  12d     v1.23.5+3afdacb
  ip-10-0-176-138.us-east-2.compute.internal  Ready                       master  12d     v1.23.5+3afdacb
  ip-10-0-183-194.us-east-2.compute.internal  Ready                       worker  12d     v1.23.5+3afdacb
  ip-10-0-204-102.us-east-2.compute.internal  Ready                       master  12d     v1.23.5+3afdacb
  ip-10-0-207-224.us-east-2.compute.internal  Ready                       worker  12d     v1.23.5+3afdacb

  If the status of the node is NotReady or SchedulingDisabled, then the node is not available and this impacts the update duration.

  You can check the status of nodes from the Administrator perspective in the web console by expanding Compute → Node.

Additional resources

• Machine config overview

• Pod disruption budget

Estimating cluster update time

Historical update duration of similar clusters provides you the best estimate for the future cluster updates. However, if the historical data is not available, you can use the following convention to estimate your cluster update time:

Cluster update time = CVO target update payload deployment time + (# node update iterations x MCO node update time)

A node update iteration consists of one or more nodes updated in parallel. The control plane nodes are always updated in parallel with the compute nodes. In addition, one or more compute nodes can be updated in parallel based on the maxUnavailable value.

For example, to estimate the update time, consider an OKD cluster with three control plane nodes and six compute nodes and each host takes about 5 minutes to reboot.

Scenario-1

When you set maxUnavailable to 1 for both the control plane and compute nodes Machine Config Pool (MCP), then all the six compute nodes will update one after another in each iteration:

Cluster update time = 60 + (6 x 5) = 90 minutes

Scenario-2

When you set maxUnavailable to 2 for the compute node MCP, then two compute nodes will update in parallel in each iteration. Therefore it takes total three iterations to update all the nodes.

Cluster update time = 60 + (3 x 5) = 75 minutes

Fedora compute nodes

Fedora compute nodes require an additional usage of openshift-ansible to update node binary components. The actual time spent updating Fedora compute nodes should not be significantly different from Fedora CoreOS (FCOS) compute nodes.

Additional resources

• Updating RHEL compute machines