I have deployed the Prometheus operator with Kube-state-metric. I wanted to store the metrics for just 30 minutes period of time since the data is stored in the in-memory storage. I don't have any PVC attached to the pod. I have set a retention period for the '30s'. But Prometheus is not deleting the old metrics after 30 minutes.

apiVersion: monitoring.coreos.com/v1
kind: Prometheus
metadata:
  labels:
    prometheus: k8s
  name: k8s
  namespace: monitoring
spec:
    beta.kubernetes.io/os: linux
  replicas: 3
  resources:
    limits:
      memory: 300Mi
      cpu: 300m
    requests:
      memory: 200Mi
      cpu: 200m
  ruleSelector:
    matchLabels:
      prometheus: k8s
      role: alert-rules
  securityContext:
    fsGroup: 2000
    runAsNonRoot: true
    runAsUser: 1000
  serviceAccountName: prometheus-k8s
  logLevel: info
  retention: 30m
  storage:
    emptyDir: {}
  version: v2.5.0