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

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

Let me add a few details to what was already said both in comments and in Jonas's answer.
Inferring from the comments, you've not defined a StorageClass named elasticdata. If it doesn't exist, you cannot reference it in your PV and PVC.
Take a quick look at how hostPath is used to define a PersistentVolume and how it is referenced in a PersistentVolumeClaim. Here you can see that in the example storageClassName: manual is used. Kubernetes docs doesn't say it explicitely but if you take a look at Openshift docs, it says very clearly that:
A Pod that uses a hostPath volume must be referenced by manual
(static) provisioning.
It's not just some value used to bind PVC request to this specific PV. So if the elasticdata StorageClass hasn't been defined, you should't use it here.
Second thing. As Jonas already stated in his comment, there is one-to-one binding between PVC and PV so no matter that your PV still has enough capacity, it has been already claimed by a different PVC and is not available any more. As you can read in the official docs:
A PVC to PV binding is a one-to-one mapping, using a ClaimRef which is
a bi-directional binding between the PersistentVolume and the
PersistentVolumeClaim.
Claims will remain unbound indefinitely if a matching volume does not
exist. Claims will be bound as matching volumes become available. For
example, a cluster provisioned with many 50Gi PVs would not match a
PVC requesting 100Gi. The PVC can be bound when a 100Gi PV is added to
the cluster.
And vice versa. If there is just one 100Gi PV it won't be able to safisfy a request from two PVCs claiming for 50Gi each. Note that in the result of kubectl get pv,pvc you posted, both PV and PVC have capacity of 20Gi although you request in each PVC created from PVC template only 3Gi.
You don't work here with any dynamic storage provisioner so you need to provide manually as many PersistentVolumes as needed for your use case.
By the way, instead of using hostPath I would rather recommend you using local volume with properly defined StorageClass. It has a few advantages over HostPath. Additionally an external static provisioner can be run separately for improved management of the local volume lifecycle

Answer 2

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.