Jacob Tomlinson
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Debugging Kubernetes PVCs

2 minute read #kubernetes, #persistent-volume-claim, #debugging Archive

This is an archived post that was originally published on IT Next .

Sometimes I find that something goes wrong in a container and some data stored in a persistent volume gets corrupted. This may result in me having to get my hands dirty and have a poke around in the filesystem myself.

Recently I tried to do this with a Prometheus container and found that there was no shell or environment inside the container (à la best practice!). This meant I needed to attach the PVC to a different pod with an environment I could use to debug.

Detach the volume

The container was stuck in a restart loop because of the corrupted data. So the first step was to scale the deployment to zero.

$ kubectl scale deployment my-deployment --replicas=0
deployment.extensions "my-deployment" scaled

Create a debugging pod

Now I need to inspect the deployment to find out which PVC I want to explore.

$ kubectl describe deployment my-deployment | grep ClaimName
ClaimName:  my-claim

Then I want to create a new pod spec which mounts the same PVC but using a different docker image. In this example I’ll use busybox as I just need a basic shell, but you could use any debugging tools image here.

# my-pvc-debugger.yaml
kind: Pod
apiVersion: v1
metadata:
  name: volume-debugger
spec:
  volumes:
    - name: volume-to-debug
      persistentVolumeClaim:
       claimName: <CLAIM NAME GOES HERE>
  containers:
    - name: debugger
      image: busybox
      command: ['sleep', '3600']
      volumeMounts:
        - mountPath: "/data"
          name: volume-to-debug

I then create this pod and run a shell inside it.

$ kubectl create -f /path/to/my-pvc-debugger.yaml
pod "volume-debugger" created
$ kubectl exec -it volume-debugger sh
/ #

Now that I’m inside the container I can explore the volume which is mounted at /data and fix the issue.

Scaling back

Once I’m happy with the volume I can exit my shell within the container and delete the debugger pod.

/ # logout
$ kubectl delete -f /path/to/my-pvc-debugger.yaml

Finally I can scale my deployment back up.

$ kubectl scale deployment my-deployment --replicas=1
deployment.extensions "my-deployment" scaled

Conclusion

In a perfect world we should never have to get hands on with our volumes, but occasionally bugs cause if to have to go and clean things up. This example shows a quick way to hop into a volume for a container which does not have any user environment.


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