Prometheus in CrashloopBackoff state with out-of-memory (OOM) error

Description

In certain situations, Prometheus pods can fail to start due to an out-of-memory (OOM) error. To identify the affected Prometheus pods, search for the last termination state that specifically relates to OOM errors.

Solution

To fix the issue, take the following steps:

1. Update argocd configmap to ignore service-monitors by updating resource.exclusions.
   1. Create a backup of the old configmap:

      kubectl get  configmap argocd-cm -n argocd -o yaml >> argocd-cm-old.yaml

   2. Edit argocd-cm:

      kubectl edit configmap argocd-cm -n argocd

   3. Add a new data entry for resource.exclusions:

      data:
        resource.exclusions: |
          - apiGroups:
              - monitoring.coreos.com
            kinds:
              - ServiceMonitor
            clusters:
              - "*"

2. Delete the envoy-stats-monitorservice monitor under theistio-system namespace:

   kubectl -n istio-system  delete servicemonitor  envoy-stats-monitor

3. Force-delete the Prometheus pods:

   kubectl delete pod  prometheus-rancher-monitoring-prometheus-0  -n cattle-monitoring-system
   kubectl delete pod  prometheus-rancher-monitoring-prometheus-1  -n cattle-monitoring-system

4. Wait for the Prometheus pods under cattle-monitoring-system to restart successfully.

Important: Deleting the the wal and chunks_head directories results in a loss of all monitoring data gathered during the days these files were accumulated. However, once Prometheus is up and running again, any new alerts and metrics data will be accessible.