Scale MySQL on Kubernetes and OpenShift¶

One of the great advantages brought by Kubernetes and the OpenShift platform is the ease of an application scaling. Scaling an application results in adding resources or Pods and scheduling them to available Kubernetes nodes.

Scaling can be vertical and horizontal. Vertical scaling adds more compute or storage resources to MySQL nodes; horizontal scaling is about adding more nodes to the cluster.

Vertical scaling¶

Scale compute¶

There are multiple components that Operator deploys and manages: Percona XtraDB Cluster (PXC), HAProxy or ProxySQL, etc. To add or reduce CPU or Memory you need to edit corresponding sections in the Custom Resource. We follow the structure for requests and limits that Kubernetes provides.

To add more resources to your MySQL nodes in PXC edit the following section in the Custom Resource:

spec:
  pxc:
    resources:
      requests: 
        memory: 4G
        cpu: 2
      limits:
        memory: 4G
        cpu: 2

Use our reference documentation for the Custom Resource options for more details about other components.

Scale storage¶

Kubernetes manages storage with a PersistentVolume (PV), a segment of storage supplied by the administrator, and a PersistentVolumeClaim (PVC), a request for storage from a user. In Kubernetes v1.11 the feature was added to allow a user to increase the size of an existing PVC object. The user cannot shrink the size of an existing PVC object.

Volume Expansion capability¶

Certain volume types support PVCs expansion (exact details about PVCs and the supported volume types can be found in Kubernetes documentation).

You can run the following command to check if your storage supports the expansion capability:

$ kubectl describe sc <storage class name> | grep allowVolumeExpansion

Expected output

allowVolumeExpansion: true

Get the list of volumes for you cluster:

$ kubectl get pvc -l app.kubernetes.io/instance=<CLUSTER_NAME>

Expected output

NAME                     STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
datadir-cluster1-pxc-0   Bound    pvc-90f0633b-0938-4b66-a695-556bb8a9e943   6Gi        RWO            standard       5m13s
datadir-cluster1-pxc-1   Bound    pvc-7409ea83-15b6-448f-a6a0-12a139e2f5cc   6Gi        RWO            standard       3m52s
datadir-cluster1-pxc-2   Bound    pvc-90f0b2f8-9bba-4262-904c-1740fdd5511b   6Gi        RWO            standard       2m40s

Patch the volume to increase the size

You can either edit the pvc or run the patch command:

$ kubectl patch pvc <pvc-name> -p '{ "spec": { "resources": { "requests": { "storage": "NEW STORAGE SIZE" }}}}'

Expected output

persistentvolumeclaim/datadir-cluster1-pxc-0 patched

Check if expansion is successful by running describe:

$ kubectl describe pvc <pvc-name>

Expected output

...
Normal  ExternalExpanding           3m52s              volume_expand                                                                                     CSI migration enabled for kubernetes.io/gce-pd; waiting for external resizer to expand the pvc
Normal  Resizing                    3m52s              external-resizer pd.csi.storage.gke.io                                                            External resizer is resizing volume pvc-90f0633b-0938-4b66-a695-556bb8a9e943
Normal  FileSystemResizeRequired    3m44s              external-resizer pd.csi.storage.gke.io                                                            Require file system resize of volume on node
Normal  FileSystemResizeSuccessful  3m10s              kubelet                                                                                           MountVolume.NodeExpandVolume succeeded for volume "pvc-90f0633b-0938-4b66-a695-556bb8a9e943"

Repeat step 2 for all the volumes of your cluster.

Now we have increased storage, but our StatefulSet and Custom Resource are not in sync. Edit your Custom Resource with new storage settings and apply:
```
spec:
  pxc:
    volumeSpec:
      persistentVolumeClaim:
        resources:
          requests:
            storage: <NEW STORAGE SIZE>
```
Apply the Custom Resource:
```
$ kubectl apply -f cr.yaml
```
Delete the StatefulSet to syncronize it with Custom Resource:
```
$ kubectl delete sts <statefulset-name> --cascade=orphan
```
The Pods will not go down and Operator is going to recreate the StatefulSet:
```
$ kubectl get sts <statefulset-name>
```
Expected output
```
cluster1-pxc       3/3     39s
```

No Volume Expansion capability¶

Scaling the storage without Volume Expansion is also possible. We will need to delete Pods one by one and their persistent volumes to resync the data to the new volumes. This can also be used to shrink the storage.

Edit the Custom Resource with the new storage size as follows:

spec:
  pxc:
    volumeSpec:
      persistentVolumeClaim:
        resources:
          requests:
            storage: <NEW STORAGE SIZE>

Apply the Custom Resource update in a usual way:

$ kubectl apply -f deploy/cr.yaml

Delete the StatefulSet with the orphan option
```
$ kubectl delete sts <statefulset-name> --cascade=orphan
```
The Pods will not go down and the Operator is going to recreate the StatefulSet:
```
$ kubectl get sts <statefulset-name>
```
Expected output
```
cluster1-pxc       3/3     39s
```

Scale up the cluster (Optional)

Changing the storage size would require us to terminate the Pods, which decreases the computational power of the cluster and might cause performance issues. To improve performance during the operation we are going to change the size of the cluster from 3 to 5 nodes:

...
spec:
  pxc:
    size: 5

Apply the change:

$ kubectl apply -f deploy/cr.yaml

New Pods will already have new storage:

$ kubectl get pvc

Expected output

NAME                     STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
datadir-cluster1-pxc-0   Bound    pvc-90f0633b-0938-4b66-a695-556bb8a9e943   10Gi       RWO            standard       110m
datadir-cluster1-pxc-1   Bound    pvc-7409ea83-15b6-448f-a6a0-12a139e2f5cc   10Gi       RWO            standard       109m
datadir-cluster1-pxc-2   Bound    pvc-90f0b2f8-9bba-4262-904c-1740fdd5511b   10Gi       RWO            standard       108m
datadir-cluster1-pxc-3   Bound    pvc-439bee13-3b57-4582-b342-98281aca50ba   19Gi       RWO            standard       49m
datadir-cluster1-pxc-4   Bound    pvc-2d4f3a60-4ec4-48a0-96cd-5243e2f05234   19Gi       RWO            standard       47m

Delete PVCs and Pods with old storage size one by one. Wait for data to sync before you proceeding to the next node.
```
$ kubectl delete pvc <PVC NAME>
$ kubectl delete pod <POD NAME>
```
The new PVC is going to be created along with the Pod.

Horizontal scaling¶

Size of the cluster is controlled by a size key in the Custom Resource options configuration. That’s why scaling the cluster needs nothing more but changing this option and applying the updated configuration file. This may be done in a specifically saved config:

...
spec:
  pxc:
    size: 5

Apply the change:

$ kubectl apply -f deploy/cr.yaml

Alternatively, you cana do it on the fly, using the following command:

$ kubectl scale --replicas=5 pxc/<CLUSTER NAME>

In this example we have changed the size of the Percona XtraDB Cluster to 5 instances.

Automated scaling¶

To automate horizontal scaling it is possible to use Horizontal Pod Autoscaler (HPA). It will scale the Custom Resource itself, letting Operator to deal with everything else.

It is also possible to use Kuvernetes Event-driven Autoscaling (KEDA), where you can apply more sophisticated logic for decision making on scaling.

For now it is not possible to use Vertical Pod Autoscaler (VPA) with the Operator due to the limitations it introduces for objects with owner references.

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Last update: 2023-10-12