Demo Storage

Welcome — this demo covers the basics of using storage in an OpenShift cluster. You will see how a simple Flask app can read a file from a mounted volume at /data/testfile.txt, how to inspect pod filesystems, and how to create and mount a PersistentVolumeClaim (PVC) via the OpenShift web console.

A presentation-style slide titled "Red Hat OpenShift Demo" with the heading "Storage." A vertical photo inset shows a person typing at a keyboard with code visible on a monitor.

What this app does

Renders a home page (/) that shows the pod hostname and a selected color.
Changes the displayed color via the route /color/<new_color>.
Reads and displays the contents of /data/testfile.txt when a user visits /read_file.

If the file /data/testfile.txt is not present (e.g., because no volume is mounted at /data), the /read_file route returns a clear error message. This is useful for verifying whether a PVC is correctly mounted into a pod.

Flask application (complete example)

Below is the corrected Flask application used in this demo. It implements the routes described above and uses with open(...) for safe file handling.

# app.py
from flask import Flask, render_template
import socket

app = Flask(__name__)

# Example color map
color_codes = {
    "red": "#ff0000",
    "green": "#00ff00",
    "blue": "#0000ff",
    "default": "#ffffff"
}

@app.route('/')
def index():
    color = color_codes.get("default")
    return render_template('hello.html', name=socket.gethostname(), color=color, contents=None)

@app.route('/color/<new_color>')
def new_color(new_color):
    color = color_codes.get(new_color, color_codes["default"])
    return render_template('hello.html', name=socket.gethostname(), color=color, contents=None)

@app.route('/read_file')
def read_file():
    contents = None
    try:
        with open("/data/testfile.txt", "r", encoding="utf-8") as f:
            contents = f.read()
    except FileNotFoundError:
        contents = "Error: /data/testfile.txt not found on this pod."
    except Exception as e:
        contents = f"Error reading file: {e}"
    return render_template('hello.html', name=socket.gethostname(), color=color_codes["default"], contents=contents)

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

Color mapping quick reference

Color name	Hex code
red	`#ff0000`
green	`#00ff00`
blue	`#0000ff`
default	`#ffffff`

Inspecting the pod filesystem

If /data is not mounted or the file does not exist, you can open a terminal for the pod in the OpenShift web console and inspect the root filesystem and mounts. Example root directory listing from inside a container:

(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 May   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-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$

If /data is missing or empty, /read_file will fail with the FileNotFound error message returned by the app.

Add persistent storage with the OpenShift web console

To mount persistent storage at /data so the app can read /data/testfile.txt, follow these steps in the OpenShift web console:

Go to the Storage section and click Create Storage.
Create a PersistentVolumeClaim (PVC): provide a name and the requested size.
Open your DeploymentConfig (or Deployment) and click Add Storage under Volumes.
Select the PVC you created and set the Container Mount Path to /data.
Redeploy the workload so the pod mounts the PVC.

After redeploying, open the pod terminal and verify /data is present and writable. Example commands you can run inside the pod to create and verify the test file:

# inside the pod terminal
(app-root) sh-4.2$ mkdir -p /data
(app-root) sh-4.2$ cat > /data/testfile.txt <<'EOF'
This is test contents
This is more contents
EOF
(app-root) sh-4.2$ cat /data/testfile.txt
This is test contents
This is more contents

Now visit http://<route-or-pod>/read_file in your browser — the app will read /data/testfile.txt and display its contents. If you edit the file inside the pod and refresh the page, the new contents will be displayed.

Behavior when scaling replicas

If you scale your application (increase replica count), each replica that mounts the same PVC at /data will see the same testfile.txt.
Whether multiple pods can mount the same PVC concurrently depends on the storage class/provisioner and the PVC access mode (e.g., ReadWriteOnce vs ReadWriteMany).

Make sure your storage class and PVC support the desired access mode (for example, ReadWriteMany if you need multiple pods to concurrently mount the same volume). Check the storage class documentation or your cloud provider’s guidance when configuring shared storage.

Troubleshooting checklist

Confirm the pod shows the PVC mounted at /data.
Verify the file permissions allow the container user to read /data/testfile.txt.
If FileNotFoundError appears, ensure the PVC is bound and the mount path is correct.
Check pod events and deployment rollout logs for volume-related errors.

Links and references

OpenShift Storage concepts: https://docs.openshift.com/ (search “OpenShift persistent storage” in the docs)
Kubernetes Persistent Volumes and PersistentVolumeClaims: https://kubernetes.io/docs/concepts/storage/persistent-volumes/
Storage class access modes (ReadWriteOnce, ReadWriteMany): https://kubernetes.io/docs/concepts/storage/persistent-volumes/#access-modes

Thanks for following this demo — you should now be able to mount a PVC into an OpenShift pod and serve file contents from /data/testfile.txt in your application.

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

Conclusion and APPENDIX

What this app does

Flask application (complete example)

Color mapping quick reference

Inspecting the pod filesystem

Add persistent storage with the OpenShift web console

Behavior when scaling replicas

Troubleshooting checklist

Links and references

Watch Video

​What this app does

​Flask application (complete example)

​Color mapping quick reference

​Inspecting the pod filesystem

​Add persistent storage with the OpenShift web console

​Behavior when scaling replicas

​Troubleshooting checklist

​Links and references

Watch Video

What this app does

Flask application (complete example)

Color mapping quick reference

Inspecting the pod filesystem

Add persistent storage with the OpenShift web console

Behavior when scaling replicas

Troubleshooting checklist

Links and references