kubernetes - Statefulset with replicas : 1 pod has unbound immediate PersistentVolumeClaims

Question

I'm trying to setup , in my single node cluster (Docker Desktop Windows), an elastic cluster. For this, I have created the PV as followed (working)

apiVersion: v1
kind: PersistentVolume
metadata:
  name: elastic-pv-data
  labels:
    type: local
spec:
  storageClassName: elasticdata
  accessModes:   
    - ReadWriteOnce
  capacity:
    storage: 20Gi
  hostPath:
    path: "/mnt/data/elastic"

Then here is the configuration :

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: esnode
spec:
  selector:
    matchLabels:
      app: es-cluster # has to match .spec.template.metadata.labels
  serviceName: elasticsearch
  replicas: 2
  updateStrategy:
    type: RollingUpdate
  template:
    metadata:
      labels:
        app: es-cluster
    spec:
      securityContext:
        fsGroup: 1000
      initContainers:
      - name: init-sysctl
        image: busybox
        imagePullPolicy: IfNotPresent
        securityContext:
          privileged: true
        command: ["sysctl", "-w", "vm.max_map_count=262144"]
      containers:
      - name: elasticsearch
        resources:
            requests:
                memory: 1Gi
        securityContext:
          privileged: true
          runAsUser: 1000
          capabilities:
            add:
            - IPC_LOCK
            - SYS_RESOURCE
        image: docker.elastic.co/elasticsearch/elasticsearch-oss:7.7.1
        env:
        - name: ES_JAVA_OPTS
          valueFrom:
              configMapKeyRef:
                  name: es-config
                  key: ES_JAVA_OPTS
        readinessProbe:
          httpGet:
            scheme: HTTP
            path: /_cluster/health?local=true
            port: 9200
          initialDelaySeconds: 5
        ports:
        - containerPort: 9200
          name: es-http
        - containerPort: 9300
          name: es-transport
        volumeMounts:
        - name: es-data
          mountPath: /usr/share/elasticsearch/data
  volumeClaimTemplates:
    - metadata:
        name: es-data
      spec:
        storageClassName: elasticdata
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 3Gi

And the result is only one "pod" has its pvc binded to the pv, the other one gets an error loop "0/1 nodes are available: 1 pod has unbound immediate PersistentVolumeClaims". Here is the kubectl get pv,pvc result :

NAME                               CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                      STORAGECLASS   REASON   AGE
persistentvolume/elastic-pv-data   20Gi       RWO            Retain           Bound    default/es-data-esnode-0   elasticdata             14m

NAME                                     STATUS   VOLUME            CAPACITY   ACCESS MODES   STORAGECLASS   AGE
persistentvolumeclaim/es-data-esnode-0   Bound    elastic-pv-data   20Gi       RWO            elasticdata    13m

If I undestood correctly, I should have a second persistantolumeclaim with the following identifier : es-data-esnode-1 Is there something I miss or do not understand correctly ? Thanks for your help

I skip here the non relevant parts (configmap,loadbalancer,..)

Answer 1

When using a StatefulSet with volumeClaimTemplates, it will create a PersistentVolumeClaim for each replica. So if you use replicas: 2, two different PersistentVolumeClaims will be created, es-data-esnode-0 and es-data-esnode-1.

Each PersistentVolumeClaim will bound to an unique PersistentVolume, so in the case of two PVCs, you would need two different PersistentVolumes. But this is not easy to do using volumeClaimTemplate and hostPath-volumes in a desktop setup.

For what reasons do you need replicas: 2 in this case? It is usually used to provide better availability, e.g. using more than one node. But for a local setup in a desktop environment, usually a single replica on the single node should be fine? I think the easies solution for you is to use replicas: 1.