Demo Storage

Welcome to this demo showing how to add persistent storage to an OpenShift 4 application. This lesson covers creating a PersistentVolumeClaim (PVC) from the OpenShift web console, mounting it into your DeploymentConfig, and verifying the application can read and update files stored on the mounted volume. This demo is part of the OpenShift 4 course.

Goal

Add persistent storage to an application so the app can read a file at /data/testfile.txt from within the container and reflect updates across replicas when the underlying storage supports shared mounts.

Application used in this demo

I updated a simple Flask application so it reads a file and displays its contents on the /read_file route. The app expects the file at /data/testfile.txt. Here is the simplified Flask app used for this demo:

from flask import Flask, render_template
import socket

app = Flask(__name__)

color_codes = {
    'red': '#ff0000',
    'green': '#00ff00',
    'blue': '#0000ff'
}
color = 'green'

@app.route('/')
def index():
    return render_template('hello.html', name=socket.gethostname(), color=color_codes[color])

@app.route('/color/<new_color>')
def new_color(new_color):
    return render_template('hello.html', name=socket.gethostname(), color=color_codes[new_color])

@app.route('/read_file')
def read_file():
    try:
        with open("/data/testfile.txt", "r") as f:
            contents = f.read()
    except FileNotFoundError:
        contents = "File not found: /data/testfile.txt"
    return render_template('hello.html', name=socket.gethostname(), contents=contents)

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080)

Reproducing the issue (before adding storage)

If you try to open /read_file before adding storage, the app returns an error because /data/testfile.txt (or even the /data directory) does not exist inside the pod. A quick way to confirm this is to open a terminal into the pod (from the OpenShift web console, select the pod and click Terminal) and list the root filesystem and mounts. The terminal is a useful tool for inspections and debugging. Example terminal session showing the root filesystem inside the pod:

(app-root) sh-4.2$ ls /
anaconda-post.log  bin  boot  dev  etc  help.1  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
(app-root) sh-4.2$ ls -l /
total 72
-rw-r--r--    1 root     root       11976 Apr  2 18:39 anaconda-post.log
lrwxrwxrwx    1 root     root           7 Apr  2 18:38 bin -> usr/bin
drwxr-xr-x    3 root     root        4096 Apr 18 14:56 boot
drwxr-xr-x    5 root     root         380 May  1 12:39 dev
drwxr-xr-x   83 root     root        4096 May  1 12:39 etc
-rw-r--r--    1 root     root         9994 Apr 18 14:56 help.1
drwxr-xr-x    2 root     root        4096 Nov  5  2016 home
lrwxrwxrwx    1 root     root           7 Apr  2 18:38 lib -> usr/lib
lrwxrwxrwx    1 root     root           9 Apr  2 18:38 lib64 -> usr/lib64
drwxr-xr-x    2 root     root        4096 Nov  5  2016 media
drwxr-xr-x    2 root     root        4096 Nov  5  2016 mnt
drwxr-xr-x   13 root     root        4096 May  1 12:39 opt
dr-xr-xr-x  204 root     root           0 May  1 12:39 proc
dr-xr-xr-x    2 root     root        4096 Apr  2 18:39 root
drwxr-xr-x   13 root     root        4096 May  1 12:39 run
lrwxrwxrwx    1 root     root           8 Apr  2 18:38 sbin -> usr/sbin
drwxr-xr-x    2 root     root        4096 Nov  5  2016 srv
dr-xr-xr-x   13 root     root           0 May  1 02:01 sys
drwxrwxrwt    9 root     root        4096 May  1 12:15 tmp
drwxr-xr-x   42 root     root        4096 Apr 11 08:07 usr
drwxr-xr-x   34 root     root        4096 Apr  2 18:38 var
(app-root) sh-4.2$

Add persistent storage (PVC) via OpenShift web console

To persist the file across pod restarts and share it between replicas (when supported), create a PersistentVolumeClaim (PVC). From the OpenShift web console:

Navigate to Storage → Create PersistentVolumeClaim.
Provide a claim name, requested size, and select the appropriate access mode (for example RWO or RWX depending on your storage backend).

A screenshot of the OpenShift Origin web console showing the "Create Storage" form, with fields for a storage claim name, access mode (RWO/RWX/ROX) and size. The left navigation pane highlights Storage and Create/Cancel buttons are visible at the bottom.

Once the PVC is created, edit your application’s DeploymentConfig and click Add Storage under Volumes. Set the mount path inside the container to /data (this matches the Flask app’s expected path). After updating the DeploymentConfig, redeploy or let OpenShift rollout the new pods. Then open a terminal into one of the new pods and verify /data is now a mounted volume:

# inside the pod terminal
ls -l /data
mount | grep /data

Create testfile.txt in the mounted volume and add some content:

This is test contents
This is more contents

Now access the application at /read_file. The UI should display the text from /data/testfile.txt. If you append more text to /data/testfile.txt and refresh the browser, the application will show the updated contents.

Multiple replicas and shared storage

If you scale the application, new replicas will mount the same PVC. Whether multiple pods can read or write to that volume concurrently depends on the PVC’s access mode and the underlying storage provider.

Access Mode	Description	Typical Use Case
ReadWriteOnce (RWO)	Volume can be mounted read-write by a single node	Single-writer deployments, or when using block/storage that only allows one attach at a time
ReadWriteMany (RWX)	Volume can be mounted read-write by many nodes	Shared read/write access across multiple pods/replicas (e.g., NFS, GlusterFS, some cloud file systems)
ReadOnlyMany (ROX)	Volume can be mounted read-only by many nodes	Shared read-only data across multiple pods

Use the appropriate access mode when creating your PVC depending on whether replicas need simultaneous write access.

If multiple replicas require simultaneous read/write access, make sure your storage class and PersistentVolume support ReadWriteMany (RWX). Many backend volumes only support ReadWriteOnce (RWO), which restricts writable mounts to a single node.

Quick verification checklist

PVC created in OpenShift and bound to a PV.
DeploymentConfig updated to mount the PVC at /data.
New pods show /data as a mounted filesystem.
testfile.txt created at /data/testfile.txt.
Accessing /read_file in the app displays the file contents.
If running multiple replicas, confirm the storage supports the chosen access mode (RWX vs RWO).

Introduction

Pre-Requisite – Docker and Kubernetes

Getting Started with OpenShift

OpenShift Concepts – Projects and Users

Concepts – Builds and Deployments

Networks, Services, Routes and Scaling

Storage, Templates and Catalog

OpenShift Security

Conclusion and APPENDIX

Goal

Application used in this demo

Reproducing the issue (before adding storage)

Add persistent storage (PVC) via OpenShift web console

Multiple replicas and shared storage

Quick verification checklist

Links and references

Watch Video

​Goal

​Application used in this demo

​Reproducing the issue (before adding storage)

​Add persistent storage (PVC) via OpenShift web console

​Multiple replicas and shared storage

​Quick verification checklist

​Links and references

Watch Video

Goal

Application used in this demo

Reproducing the issue (before adding storage)

Add persistent storage (PVC) via OpenShift web console

Multiple replicas and shared storage

Quick verification checklist

Links and references