Non-stop storage scaling (vertical or horizontal scaling) is essential in a data-intensive system, database servers in particular.
Is it possible in Kubernetes?
In Kubernetes v1.11 the persistent volume expansion feature is being promoted to beta and enabled by default.1 There is also a nontrivial real-world use case for this: Strimzi Kafka operator. Strimzi merely builds on top of existing Kubernetes storage class mechanism to grow the storage of Kafka cluster.2 Therefore, the volume resizing feature is not exclusively available for Strimzi and Kafka. You can put this feature into your own application as long as you learn the mechanism.
To get a concrete knowledge of the volume resizing, I’ve conducted a simple experiment on GCP and GKE. The experiment tries to answer the following questions:
- Does volume resizing really work in Kubernetes?
- Is the resizing process non-stop?
- Are data still persistent after resizing?
All experiment materials are available in the vol-resize repo, for your convenience.
About the sample app
A sample app voltest.sh will be used for the whole experiment.3 It displays the size of available disk spaces in the specified mount point (as the 1st argument), and also records the data continuously in the specified output file (as the 2nd argument; default to
Let’s see how it works. Assume that we are in the
/home directory, with available disk space about 4.5 GB:
% pwd /home % % df -h Filesystem Size Used Avail Use% Mounted on overlay 41G 32G 8.2G 80% / tmpfs 64M 0 64M 0% /dev tmpfs 848M 0 848M 0% /sys/fs/cgroup /dev/sda1 41G 32G 8.2G 80% /root /dev/sdb1 4.8G 38M 4.5G 1% /home overlayfs 1.0M 160K 864K 16% /etc/ssh/keys shm 64M 0 64M 0% /dev/shm overlayfs 1.0M 160K 864K 16% /etc/ssh/ssh_host_dsa_key tmpfs 848M 736K 847M 1% /run/metrics tmpfs 848M 0 848M 0% /run/google/devshell
At first run, it shows the space of current directory and records them continuously in the
data file within the same directory:
% docker run -it -v $(pwd):/mnt williamyeh/voltest /mnt ---> Checking /mnt/data 1 : 4.5G 2 : 4.5G 3 : 4.5G 4 : 4.5G 5 : 4.5G 6 : 4.5G 7 : 4.5G 8 : 4.5G 9 : 4.5G 10 : 4.5G 11 : 4.5G 12 : 4.5G ^C %
Interrupt the execution at 12 on purpose, and run again. You’ll see that it picks up the last serial number 12 and continues counting:
% docker run -it -v $(pwd):/mnt williamyeh/voltest /mnt ---> Checking /mnt/data 13 : 4.5G 14 : 4.5G 15 : 4.5G 16 : 4.5G 17 : 4.5G 18 : 4.5G 19 : 4.5G 20 : 4.5G 21 : 4.5G 22 : 4.5G 23 : 4.5G ^C %
data file should have 23 lines of records:
% cat data 1 : 4.5G 2 : 4.5G 3 : 4.5G 4 : 4.5G 5 : 4.5G 6 : 4.5G 7 : 4.5G 8 : 4.5G 9 : 4.5G 10 : 4.5G 11 : 4.5G 12 : 4.5G 13 : 4.5G 14 : 4.5G 15 : 4.5G 16 : 4.5G 17 : 4.5G 18 : 4.5G 19 : 4.5G 20 : 4.5G 21 : 4.5G 22 : 4.5G 23 : 4.5G %
In the following experiment we’ll use the app and the data file to answer the 3 questions:
Does volume resizing really work in Kubernetes? Just inspect the disk spaces it displays.
Is the resizing process non-stop? Just inspect the screen output.
Are data still persistent after resizing? Just inspect the content of data file.
Experiment part 1: initial size
The experiment is conducted on GCP and GKE. However, it should apply to other cloud Kubernetes platforms as well, with minor modification.
➊ Clone the experiment repo to your workspace or Cloud Shell:
% git clone https://github.com/William-Yeh/vol-resize.git
➋ Prepare a Kubernetes cluster in GKE.
➌ Create a 20GB persistent disk named
voltest. For example, the following command will create such a persistent disk in the us-central1-a zone:
% gcloud compute disks create \ --size=20GB --zone=us-central1-a \ voltest
Check if the underlying persistent disk is created:
➍ We need a storage class with
allowVolumeExpansion enabled. The manifest file expansion-ss.yml is provided as follows:
apiVersion: storage.k8s.io/v1 kind: StorageClass metadata: name: expansion parameters: type: pd-standard provisioner: kubernetes.io/gce-pd allowVolumeExpansion: true reclaimPolicy: Delete
Create a storage class
expansion for this:
% kubectl apply -f expansion-ss.yml
➎ We need a PV manifest file voltest-pv.yml to associate it with the existing persistent disk just created before:
apiVersion: v1 kind: PersistentVolume metadata: name: voltest spec: storageClassName: "expansion" accessModes: - ReadWriteOnce capacity: storage: 20Gi gcePersistentDisk: pdName: voltest fsType: ext4
Create the persistent volume
% kubectl apply -f voltest-pv.yml persistentvolume/voltest created % kubectl get pv NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE voltest 20Gi RWO Retain Available expansion 8s
Note that the
voltest PV is in “Available” status.
➏ We need a PVC manifest file voltest-pvc.yml to claim the PV:
apiVersion: v1 kind: PersistentVolumeClaim metadata: name: voltest spec: volumeName: voltest storageClassName: "expansion" accessModes: - ReadWriteOnce resources: requests: storage: 20Gi
Claim the persistent volume now:
% kubectl apply -f voltest-pvc.yml persistentvolumeclaim/voltest created % kubectl get pvc NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE voltest Bound voltest 20Gi RWO expansion 11s % kubectl get pv NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE voltest 20Gi RWO Retain Bound default/voltest expansion 13m
Note that the
voltest PV/PVC pair is in “Bound” status.
➐ We need a manifest file voltest-app.yml for our sample app to access the PV/PVC:
apiVersion: apps/v1 kind: Deployment ... spec: replicas: 1 ... template: # pod definition ... spec: containers: - name: voltest image: williamyeh/voltest volumeMounts: - mountPath: "/mnt" name: voltest volumes: - name: voltest persistentVolumeClaim: claimName: voltest
Invoke the sample app now:
% kubectl apply -f voltest-app.yml
Open another terminal pane to watch the logs continuously:
% kubectl logs -f deployment/voltest ---> Checking /mnt/data 1 : 20G 2 : 20G 3 : 20G 4 : 20G 5 : 20G 6 : 20G 7 : 20G 8 : 20G 9 : 20G 10 : 20G 11 : 20G 12 : 20G ...
Quite familiar output. Everything works fine.
Keep the logs running. We’ll see it again and again.
Experiment part 2: resizing
Now we’re about to resize the volume.
➊ Edit the
% kubectl edit pvc/voltest
➋ Change the value of
100Gi. Save, and exit.
➌ Check if the logs are still being generated, and the content of the logs.
➍ Check the size of underlying persistent disk. It should be expanded to 100 GB now.
➎ Check if the PV/PVC are both expanded:
% kubectl get pv NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE voltest 100Gi RWO Retain Bound default/voltest expansion 45m % kubectl get pvc NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE voltest Bound voltest 20Gi RWO expansion 32m
PV is expanded to 100 GB, but PVC not yet. In other words, block storage volume is expanded, but file system is not yet. The reason is described in the “Resizing Persistent Volumes using Kubernetes” article:
“Once underlying volume has been expanded by the storage provider, then the PersistentVolume object will reflect the updated size and the PVC will have the
“File system expansion must be triggered by terminating the pod using the volume […] then pod that uses the PVC can be restarted to finish file system resizing on the node.”
For now the question “2. Is the resizing process non-stop?" should be obviously answered. Let’s move on to handle the
FileSystemResizePending condition by restarting the related pods.
Experiment part 3: restart the pod
➊ Kill the pod, and let deployment restart a new pod for us (
% kubectl get pods NAME READY STATUS RESTARTS AGE voltest-d88ff8c49-66wk2 1/1 Running 0 23m % kubectl delete pod voltest-d88ff8c49-66wk2
➋ Watch the logs! Our sample app will see the file system expansion progress on the fly:
13465 : 20G 13466 : 20G 13467 : 20G 13468 : 24G 13469 : 30G 13470 : 30G 13471 : 30G 13472 : 30G 13473 : 30G 13474 : 42G 13475 : 77G 13476 : 99G 13477 : 99G 13478 : 99G 13479 : 99G 13480 : 99G 13481 : 99G 13482 : 99G
➌ Check if the PV/PVC are both expanded:
% kubectl get pv NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE voltest 100Gi RWO Retain Bound default/voltest expansion 57m % kubectl get pvc NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE voltest Bound voltest 100Gi RWO expansion 44m
For now the remaining questions “1. Does volume resizing really work in Kubernetes?” and “3. Are data still persistent after resizing?” should be obviously answered.
As can be seen in this experiment, all you have to do to grow the PV/PVC are:
Use Kubernetes ≥ 1.11.
Use the storage class in your PV and PVC.
Increase the PVC
Restart the related pods.
Kubernetes 1.11 also introduces an alpha feature called online file system expansion. You can track its progress in the Kubernetes CSI Developer Documentation “Volume Expansion”.