Scale Percona Server for MongoDB on Kubernetes and OpenShift¶

One of the great advantages brought by Kubernetes and the OpenShift platform is the ease of an application scaling. Scaling a Deployment up or down ensures new Pods are created and set to available Kubernetes nodes.

Scaling can be vertical and horizontal. Vertical scaling adds more compute or storage resources to MongoDB nodes; horizontal scaling is about adding more nodes to the cluster. High availability looks technically similar, because it also involves additional nodes, but the reason is maintaining liveness of the system in case of server or network failures.

Vertical scaling¶

Scale compute¶

There are multiple components that Operator deploys and manages: MongoDB replica set instances, mongos and config server instances, 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 MongoDB replica set instances, edit the following section in the Custom Resource:

spec:
  replsets:
    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 MongoDB cluster:

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

Expected output

NAME                                STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
mongod-data-my-cluster-name-cfg-0   Bound    pvc-a2b37f4d-6f11-443c-8670-de82ce9fc335   3Gi        RWO            standard       13m
mongod-data-my-cluster-name-cfg-1   Bound    pvc-ded949e5-0f93-4f57-ab2c-7c5fd9528fa0   3Gi        RWO            standard       12m
mongod-data-my-cluster-name-cfg-2   Bound    pvc-f3a441dd-94b6-4dc0-b96c-58b7851dfaa0   3Gi        RWO            standard       12m
mongod-data-my-cluster-name-rs0-0   Bound    pvc-b183c40b-c165-445a-aacd-9a34b8fff227   3Gi        RWO            standard       13m
mongod-data-my-cluster-name-rs0-1   Bound    pvc-f186426b-cbbe-4c31-860e-97a4dfca3de0   3Gi        RWO            standard       12m
mongod-data-my-cluster-name-rs0-2   Bound    pvc-6beb6ccd-8b3a-4580-b3ef-a2345a2c21d6   3Gi        RWO            standard       12m

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/mongod-data-my-cluster-name-rs0-0 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-b183c40b-c165-445a-aacd-9a34b8fff227
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-b183c40b-c165-445a-aacd-9a34b8fff227"

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:
  ...
  replsets:
    ...
    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
```
my-cluster-name-rs0       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:
  ...
  replsets:
    ...
    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

my-cluster-name-rs0       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:
  ...
  replsets:
    ...
    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
mongod-data-my-cluster-name-cfg-0   Bound    pvc-a2b37f4d-6f11-443c-8670-de82ce9fc335   10Gi       RWO            standard       110m
mongod-data-my-cluster-name-cfg-1   Bound    pvc-ded949e5-0f93-4f57-ab2c-7c5fd9528fa0   10Gi       RWO            standard       109m
mongod-data-my-cluster-name-cfg-2   Bound    pvc-f3a441dd-94b6-4dc0-b96c-58b7851dfaa0   10Gi       RWO            standard       108m
mongod-data-my-cluster-name-rs0-0   Bound    pvc-b183c40b-c165-445a-aacd-9a34b8fff227   19Gi       RWO            standard       49m
mongod-data-my-cluster-name-rs0-1   Bound    pvc-f186426b-cbbe-4c31-860e-97a4dfca3de0   19Gi       RWO            standard       47m
mongod-data-my-cluster-name-rs0-2   Bound    pvc-6beb6ccd-8b3a-4580-b3ef-a2345a2c21d6   19Gi       RWO            standard       45m

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¶

The size of the cluster is controlled by the size key in the Custom Resource options configuration.

Note

The Operator will not allow to scale Percona Server for MongoDB with the kubectl scale statefulset <StatefulSet name> command as it puts size configuration options out of sync.

You can change size separately for different components of your cluster by setting this option in the appropriate subsections:

replsets.size allows to set the size of the MongoDB Replica Set,
replsets.arbiter.size allows to set the number of Replica Set Arbiter instances,
sharding.configsvrReplSet.size allows to set the number of Config Server instances ,
sharding.mongos.size allows to set the number of mongos instances.

For example, the following update in deploy/cr.yaml will set the size of the MongoDB Replica Set to 5 nodes:

spec:
  ...
  replsets:
    ...
    size: 5

Don’t forget to apply changes as usual, running the kubectl apply -f deploy/cr.yaml command.

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Last update: 2024-04-22