2.3. Control plane node sizing
The control plane node resource requirements depend on the number of nodes in the cluster. The following control plane node size recommendations are based on the results of control plane density focused testing. The control plane tests create the following objects across the cluster in each of the namespaces depending on the node counts:
- 12 image streams
- 3 build configurations
- 6 builds
- 1 deployment with 2 pod replicas mounting two secrets each
- 2 deployments with 1 pod replica mounting two secrets
- 3 services pointing to the previous deployments
- 3 routes pointing to the previous deployments
- 10 secrets, 2 of which are mounted by the previous deployments
- 10 config maps, 2 of which are mounted by the previous deployments
| Number of worker nodes | Cluster load (namespaces) | CPU cores | Memory (GB) |
|---|---|---|---|
| 25 | 500 | 4 | 16 |
| 100 | 1000 | 8 | 32 |
| 250 | 4000 | 16 | 96 |
On a cluster with three masters or control plane nodes, the CPU and memory usage will spike up when one of the nodes is stopped, rebooted or fails because the remaining two nodes must handle the load in order to be highly available. This is also expected during upgrades because the masters are cordoned, drained, and rebooted serially to apply the operating system updates, as well as the control plane Operators update. To avoid cascading failures on large and dense clusters, keep the overall resource usage on the master nodes to at least half of all available capacity to handle the resource usage spikes. Increase the CPU and memory on the master nodes accordingly.
The node sizing varies depending on the number of nodes and object counts in the cluster. It also depends on whether the objects are actively being created on the cluster. During object creation, the control plane is more active in terms of resource usage compared to when the objects are in the running phase.
If you used an installer-provisioned infrastructure installation method, you cannot modify the control plane node size in a running OpenShift Container Platform 4.5 cluster. Instead, you must estimate your total node count and use the suggested control plane node size during installation.
The recommendations are based on the data points captured on OpenShift Container Platform clusters with OpenShiftSDN as the network plug-in.
In OpenShift Container Platform 4.5, half of a CPU core (500 millicore) is now reserved by the system by default compared to OpenShift Container Platform 3.11 and previous versions. The sizes are determined taking that into consideration.
