HostPath assign persistentVolume to the specific work node in cluster - kubernetes

Question

Using kubeadm to create a cluster, I have a master and work node.
Now I want to share a persistentVolume in the work node, which will be bound with Postgres pod.
Expecting the code will create persistentVolume in the path /postgres of work node, but it seems the hostPath will not work in a cluster, how should I assign this property to the specific node?
kind: PersistentVolume
apiVersion: v1
metadata:
name: pv-postgres
labels:
type: local
spec:
capacity:
storage: 2Gi
accessModes:
- ReadWriteOnce
hostPath:
path: "/postgres"
---
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: pvc-postgres
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: postgres
spec:
selector:
matchLabels:
app: postgres
replicas: 1
strategy: {}
template:
metadata:
labels:
app: postgres
spec:
dnsPolicy: ClusterFirstWithHostNet
hostNetwork: true
volumes:
- name: vol-postgres
persistentVolumeClaim:
claimName: pvc-postgres
containers:
- name: postgres
image: postgres:12
imagePullPolicy: Always
env:
- name: DB_USER
value: postgres
- name: DB_PASS
value: postgres
- name: DB_NAME
value: postgres
ports:
- name: postgres
containerPort: 5432
volumeMounts:
- mountPath: "/postgres"
name: vol-postgres
livenessProbe:
exec:
command:
- pg_isready
- -h
- localhost
- -U
- postgres
initialDelaySeconds: 30
timeoutSeconds: 5
readinessProbe:
exec:
command:
- pg_isready
- -h
- localhost
- -U
- postgres
initialDelaySeconds: 5
timeoutSeconds: 1
---
apiVersion: v1
kind: Service
metadata:
name: postgres
spec:
ports:
- name: postgres
port: 5432
targetPort: postgres
selector:
app: postgres

Answer 1

As per docs.
A hostPath volume mounts a file or directory from the host node’s filesystem into your Pod. This is not something that most Pods will need, but it offers a powerful escape hatch for some applications.
In short, hostPath type refers to node (machine or VM) resource, where you will schedule pod. It mean that you already need to have this folder on this node.
To assign resources to specify node you have to use nodeSelector in your Deployment, PV.
Depends on the scenario, using hostPath is not the best idea, however I will provide below example YAMLs which might show you concept. Based on your YAMLs but with nginx image.
kind: PersistentVolume
apiVersion: v1
metadata:
name: pv-postgres
spec:
capacity:
storage: 2Gi
accessModes:
- ReadWriteOnce
hostPath:
path: "/tmp/postgres" ## this folder need exist on your node. Keep in minds also who have permissions to folder. Used tmp as it have 3x rwx
nodeAffinity:
required:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- ubuntu18-kubeadm-worker1
---
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: pvc-postgres
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: postgres
spec:
selector:
matchLabels:
app: postgres
replicas: 1
strategy: {}
template:
metadata:
labels:
app: postgres
spec:
containers:
- image: nginx
name: nginx
volumeMounts:
- mountPath: /home ## path to folder inside container
name: vol-postgres
affinity: ## specified affinity to schedule all pods on this specific node with name ubuntu18-kubeadm-worker1
nodeAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- ubuntu18-kubeadm-worker1
dnsPolicy: ClusterFirstWithHostNet
hostNetwork: true
volumes:
- name: vol-postgres
persistentVolumeClaim:
claimName: pvc-postgres
persistentvolume/pv-postgres created
persistentvolumeclaim/pvc-postgres created
deployment.apps/postgres created
Unfortunately PV is bounded to PVC in 1:1 relationship, so for each time, you would need to create PV and PVC.
However if you are using hostPath it's enough to specify nodeAffinity, volumeMounts and volumes in Deployment YAML without PV and PVC.
apiVersion: apps/v1
kind: Deployment
metadata:
name: postgres
spec:
selector:
matchLabels:
app: postgres
replicas: 1
strategy: {}
template:
metadata:
labels:
app: postgres
spec:
containers:
- image: nginx:latest
name: nginx
volumeMounts:
- mountPath: /home
name: vol-postgres
affinity:
nodeAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
nodeSelectorTerms:
- matchExpressions:
- key: kubernetes.io/hostname
operator: In
values:
- ubuntu18-kubeadm-worker1
dnsPolicy: ClusterFirstWithHostNet
hostNetwork: true
volumes:
- name: vol-postgres
hostPath:
path: /tmp/postgres
deployment.apps/postgres created
user#ubuntu18-kubeadm-master:~$ kubectl get pods
NAME READY STATUS RESTARTS AGE
postgres-77bc9c4566-jgxqq 1/1 Running 0 9s
user#ubuntu18-kubeadm-master:~$ kk exec -ti postgres-77bc9c4566-jgxqq /bin/bash
root#ubuntu18-kubeadm-worker1:/# cd home
root#ubuntu18-kubeadm-worker1:/home# ls
test.txt txt.txt

Answer 2

There are ways to achieve it. You can mount your volume into a NAS or create a storage cluster using disks and create a persistent volume and persistent volume claim for that. If your use-case is to have persistence in local storage then you can create a local-storage storageclass in one of your cluster nodes and that volume space can be used by any pod in your cluster. To create a local-storage storageclass, refer this (https://kubernetes.io/blog/2019/04/04/kubernetes-1.14-local-persistent-volumes-ga/)