Introduction

In Kubernetes, A HostPath volume is a type of volume that mounts a directory or file from the host node's filesystem into a Pod. It allows a Pod to access and share files with the underlying host machine.

When you use a HostPath volume, the file or directory specified in the volume configuration is mounted directly into the container running in the Pod. This means that the container has access to the files and directories on the host machine's filesystem.

The data stored in the HostPath volume is retained even if the pod is terminated or rescheduled to a different node.

The pod has full read and write access to the files/directories on the host node's filesystem.

It can be used to share data between containers running on the same node.

Architecture Diagram

Lab

arun@Aruns-MacBook-Air ~ % kubectl get nodes
NAME       STATUS   ROLES           AGE   VERSION
master     Ready    control-plane   62d   v1.26.3
worker01   Ready    <none>          61d   v1.26.3
worker02   Ready    <none>          61d   v1.26.3

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hostpath-deployment
spec:
  replicas: 2
  selector:
    matchLabels:
      app: hostpath-app
  template:
    metadata:
      labels:
        app: hostpath-app
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - hostpath-app
            topologyKey: kubernetes.io/hostname
      containers:
      - name: my-container
        image: nginx
        volumeMounts:
        - name: hostpath-volume
          mountPath: /hostpath
      volumes:
      - name: hostpath-volume
        hostPath:
          path: /workervolume
          type: Directory

In the above example, podAntiAffinity denotes the same pod will not schedule in the same worker nodes i.e. two pods will be scheduled in two worker nodes.

Let's create the volume in both worker nodes.

root@worker01:/# mkdir /workervolume
root@worker01:/# chmod -R 777 /workervolume
root@worker01:/# cd /workervolume
root@worker01:/workervolume# touch file1 file2
root@worker01:/workervolume# ls -lrt
total 0
-rw-r--r-- 1 root root 0 Jun  1 16:51 file2
-rw-r--r-- 1 root root 0 Jun  1 16:51 file1

root@worker02:~# mkdir /workervolume
root@worker02:~# chmod -R 777 /workervolume
root@worker02:~# cd /workervolume
root@worker02:/workervolume# touch file1 file2
root@worker02:/workervolume# ls -lrt
total 0
-rw-r--r-- 1 root root 0 Jun  1 16:51 file2
-rw-r--r-- 1 root root 0 Jun  1 16:51 file1

Deploy the hostpath example

arun@Aruns-MacBook-Air kubernetes_volumes % ka hostpath_example.yaml
deployment.apps/hostpath-deployment created
arun@Aruns-MacBook-Air kubernetes_volumes %

arun@Aruns-MacBook-Air kubernetes_volumes % kgp -o wide| grep -i hostpath
hostpath-deployment-6567776857-2ftvl      1/1     Running   0              3m28s   10.244.2.107   worker02   <none>           <none>
hostpath-deployment-6567776857-d9gw6      1/1     Running   0              3m28s   10.244.1.82    worker01   <none>           <none>
arun@Aruns-MacBook-Air kubernetes_volumes %

Hostpath volumes mounted into pod

arun@Aruns-MacBook-Air kubernetes_volumes % k exec -it hostpath-deployment-6567776857-2ftvl -- bash
root@hostpath-deployment-6567776857-2ftvl:/# ls -lrt hostpath/
total 0
-rw-r--r-- 1 root root 0 Jun  1 16:51 file2
-rw-r--r-- 1 root root 0 Jun  1 16:51 file1
root@hostpath-deployment-6567776857-2ftvl:/#

Even If we delete the deployment, Volume data's will be available forever.

root@worker01:/# ls -lrt /workervolume/
total 0
-rw-r--r-- 1 root root 0 Jun  1 16:51 file2
-rw-r--r-- 1 root root 0 Jun  1 16:51 file1
root@worker01:/#

Summary

If your application needs to interact with specific files or directories on the host node, such as reading log files, accessing host-specific devices, or reading configuration files, you can use HostPath to mount those resources directly into the pod.

Instead of mounting the directory on each worker node. We can use common shared module concept called PV (Persistence Volume).

End.

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