If you are following along with this tutorial then the storage for prometheus will be on block storage as defined in the YAML settings file.

https://www.digitalocean.com/community/tutorials/how-to-set-up-digitalocean-kubernetes-cluster-monitoring-with-helm-and-prometheus-operator

# Define persistent storage for Prometheus (PVC)
prometheus:
  prometheusSpec:
    storageSpec:
      volumeClaimTemplate:
        spec:
          accessModes: ["ReadWriteOnce"]
          storageClassName: do-block-storage
          resources:
            requests:
              storage: 5Gi

# Define persistent storage for Grafana (PVC)
grafana:
  # Set password for Grafana admin user
  adminPassword: your_admin_password
  persistence:
    enabled: true
    storageClassName: do-block-storage
    accessModes: ["ReadWriteOnce"]
    size: 5Gi

# Define persistent storage for Alertmanager (PVC)
alertmanager:
  alertmanagerSpec:
    storage:
      volumeClaimTemplate:
        spec:
          accessModes: ["ReadWriteOnce"]
          storageClassName: do-block-storage
          resources:
            requests:
              storage: 5Gi

# Change default node-exporter port
prometheus-node-exporter:
  service:
    port: 30206
    targetPort: 30206

# Disable Etcd metrics
kubeEtcd:
  enabled: false

# Disable Controller metrics
kubeControllerManager:
  enabled: false

# Disable Scheduler metrics
kubeScheduler:
  enabled: false