Scaling the Cluster Monitoring Operator | Scalability and performance

Prometheus database storage requirements
Configuring cluster monitoring

OKD exposes metrics that the Cluster Monitoring Operator collects and stores in the Prometheus-based monitoring stack. As an administrator, you can view system resources, containers and components metrics in one dashboard interface, Grafana.

If you are running cluster monitoring with an attached PVC for Prometheus, you might experience OOM kills during cluster upgrade. When persistent storage is in use for Prometheus, Prometheus memory usage doubles during cluster upgrade and for several hours after upgrade is complete. To avoid the OOM kill issue, allow worker nodes with double the size of memory that was available prior to the upgrade. For example, if you are running monitoring on the minimum recommended nodes, which is 2 cores with 8 GB of RAM, increase memory to 16 GB. For more information, see BZ#1925061.

Prometheus database storage requirements

Red Hat performed various tests for different scale sizes.

The Prometheus storage requirements below are not prescriptive. Higher resource consumption might be observed in your cluster depending on workload activity and resource use.

Table 1. Prometheus Database storage requirements based on number of nodes/pods in the cluster
Number of Nodes	Number of pods	Prometheus storage growth per day	Prometheus storage growth per 15 days	RAM Space (per scale size)	Network (per tsdb chunk)
50	1800	6.3 GB	94 GB	6 GB	16 MB
100	3600	13 GB	195 GB	10 GB	26 MB
150	5400	19 GB	283 GB	12 GB	36 MB
200	7200	25 GB	375 GB	14 GB	46 MB

Approximately 20 percent of the expected size was added as overhead to ensure that the storage requirements do not exceed the calculated value.

The above calculation is for the default OKD Cluster Monitoring Operator.

CPU utilization has minor impact. The ratio is approximately 1 core out of 40 per 50 nodes and 1800 pods.

Recommendations for OKD

Use at least three infrastructure (infra) nodes.
Use at least three openshift-container-storage nodes with non-volatile memory express (NVMe) drives.

Configuring cluster monitoring

You can increase the storage capacity for the Prometheus component in the cluster monitoring stack.

Procedure

To increase the storage capacity for Prometheus:

Create a YAML configuration file, cluster-monitoring-config.yaml. For example:

apiVersion: v1
kind: ConfigMap
data:
  config.yaml: |
    prometheusK8s:
      retention: {{PROMETHEUS_RETENTION_PERIOD}} (1)
      nodeSelector:
        node-role.kubernetes.io/infra: ""
      volumeClaimTemplate:
        spec:
          storageClassName: {{STORAGE_CLASS}} (2)
          resources:
            requests:
              storage: {{PROMETHEUS_STORAGE_SIZE}} (3)
    alertmanagerMain:
      nodeSelector:
        node-role.kubernetes.io/infra: ""
      volumeClaimTemplate:
        spec:
          storageClassName: {{STORAGE_CLASS}} (2)
          resources:
            requests:
              storage: {{ALERTMANAGER_STORAGE_SIZE}} (4)
metadata:
  name: cluster-monitoring-config
  namespace: openshift-monitoring

1	A typical value is `PROMETHEUS_RETENTION_PERIOD=15d`. Units are measured in time using one of these suffixes: s, m, h, d.
2	The storage class for your cluster.
3	A typical value is `PROMETHEUS_STORAGE_SIZE=2000Gi`. Storage values can be a plain integer or as a fixed-point integer using one of these suffixes: E, P, T, G, M, K. You can also use the power-of-two equivalents: Ei, Pi, Ti, Gi, Mi, Ki.
4	A typical value is `ALERTMANAGER_STORAGE_SIZE=20Gi`. Storage values can be a plain integer or as a fixed-point integer using one of these suffixes: E, P, T, G, M, K. You can also use the power-of-two equivalents: Ei, Pi, Ti, Gi, Mi, Ki.

Add values for the retention period, storage class, and storage sizes.
Save the file.

Apply the changes by running:

$ oc create -f cluster-monitoring-config.yaml