Tutorials:Persistent volumes on the Kubernetes cluster

Persistent volumes

A persistent volume in Kubernetes is a cluster resource which can be requested by a container. For this, you have to claim a persistent volume (PV) using a persistent volume claim (PVC), which you apply in your namespace. The persistent volume claim can then be mounted to directories within a container. The important point is that the PVC survives the end of the container, i.e. the data in the PV will be permanent until the PVC is released. If the PVC is mounted again to a new container, the data will still be present. A persistent volume which is bound to a claim can not be assigned to any other claim. If the PVC is released, the PV is also released and immediately and automatically wiped clean of all data. If you want to keep your data, copy it to some other permanent storage first.

On the cluster, there are two types of persistent volumes currently configured:

• Local persistent volumes
• Host directories

Local persistent volumes should be used to import training data and store results and log files of your training. There are special PVs for monitoring the training using Tensorboard. Host directories are meant for common training data sets stored permanently on the host. They are always read only.

Local persistent volumes

These are persistent volumes which are mapped to special folders of the host filesystem of the node. Each node exposes several persistent volumes which can be claimed. The user can not control exactly which volume is bound to a claim, but can request a minimum size. A persistent volume claim for a local PV is configured like this. Code examples can be found in the subdirectory "kubernetes/example_2" of the tutorial sample code.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  # the name of the PVC, we refer to this in the container configuration
  name: tf-mnist-pvc

spec:
  resources:
    requests:
      # storage resource request. This PVC can only be bound to volumes which
      # have at least 8 GiB of storage available.
      storage: 8Gi

  # the requested storage class, see tutorial.
  storageClassName: local-ssd

  # leave these unchanged, they must match the PV type, otherwise binding will fail
  accessModes:
    - ReadWriteOnce
  volumeMode: Filesystem

The following storage classes are configured in the cluster:

When the claim is defined to your satisfaction, apply it like this:

> kubectl apply -f pvc.yml

You can check on the status of this (and every other) claim:

> kubectl get pvc
NAME           STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS   AGE
tf-mnist-pvc   Pending                                      local-ssd      11s

Since the claim has not been used by a container yet, it is not yet bound to a persitent volume (PV).

Host directories

Large training data sets which are required by many different users are stored permanently in the filesystem of several nodes. They can be claimed with a PVC as follows:

...

Reading/writing the contents of a persistent volume

You can access a PV which is bound to a PVC by mounting it into a container. For a demonstration, we use the simple container image "ubuntu:18.04", which runs a minimalistic Ubuntu, and keep it in a very long wait after container startup.

# Test pod to mount a PV bound to a PVC into a container
# Before starting this pod, apply the PVC with kubectl apply -f pvc.yaml
apiVersion: v1
kind: Pod
metadata:
  name: your-username-pvc-access-pod
spec:
  containers:
    - name: pvc-access-container

      # we use a small ubuntu base to access the PVC
      image: ubuntu:18.04
      # make sure that we have some time until the container quits by itself
      command: ['sleep', '6h']

      # list of mount paths within the container which will be
      # bound to persistent volumes.
      volumeMounts:
      - mountPath: "/mnt/pvc-mnist"
        # name of the volume for this path (from the below list)
        name: pvc-mnist

  volumes:
    # User-defined name of the persistent volume within this configuration.
    # This can be different from the name of the PVC.
    - name: pvc-mnist
      persistentVolumeClaim:
        # name of the PVC this volume binds to
        claimName: your-username-tf-mnist-pvc

After the PVC is applied, spin up the test pod with

> kubectl apply -f pvc-access-pod.yaml

You now have several options to get data to and from the container.

1. Copying data from within the container

You can get a root shell inside the container as usual (insert the correct pod name you used below):

> kubectl exec -it pvc-access-pod /bin/bash

Your pod has internet access. Thus, an option to get data to/from the pod, in particular into the persistent volume, is to use scp, which first needs to be installed inside the pod:

# sudo apt-get update && sudo apt install openssh-client
# cd /my-pvc-mount-path
# scp your.username@external-server:/path/to/data/. ./

An even better variant would be "rsync -av" instead of scp, as this only copies files which are different or do not exist in the destination. By reversing source and destination, you can also copy data out of the container this way.

2. Copying data from the outside

From the outside world, you can directly copy data to and from the container using kubectl cp, which has a very similar syntax as scp:

# to get data into the container, substitute name with correct id obtained from kubectl get pods
> kubectl cp /path/to/data/. pvc-access-pod:/my-pvc-mount/path/
# to get data from the container
> kubectl cp pvc-access-pod:/my-pvc-mount/path/. /path/to/output/

TODO: Will finish this part soon, for now, read up on Kubernetes "kubectl cp" documentation to copy stuff to/from a PV.