How to replicate a mongodb pod with its persistent storage
https://www.mongodb.com/blog/post/running-mongodb-as-a-microservice-with-docker-and-kubernetes


  MongoDB database nodes are stateful. In the event that a container
  fails, and is rescheduled, it's undesirable for the data to be lost
  (it could be recovered from other nodes in the replica set, but that
  takes time). To solve this, features such as the Volume abstraction in
  Kubernetes can be used to map what would otherwise be an ephemeral
  MongoDB data directory in the container to a persistent location where
  the data survives container failure and rescheduling.


Like @Tensibai indicated the issue is related to the replication. According to this blog it could be solved as follows.


 apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: mongo
spec:
  serviceName: "mongo"
  replicas: 3
  template:
    metadata:
      labels:
        role: mongo
        environment: test
    spec:
      terminationGracePeriodSeconds: 10
      containers:
        - name: mongo
          image: mongo
          command:
            - mongod
            - "--replSet"
            - rs0
            - "--smallfiles"
            - "--noprealloc"
          ports:
            - containerPort: 27017
          volumeMounts:
            - name: mongo-persistent-storage
              mountPath: /data/db
        - name: mongo-sidecar
          image: cvallance/mongo-k8s-sidecar
          env:
            - name: MONGO_SIDECAR_POD_LABELS
              value: "role=mongo,environment=test"
  volumeClaimTemplates:
  - metadata:
      name: mongo-persistent-storage
      annotations:
        volume.beta.kubernetes.io/storage-class: "fast"
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 100Gi