Expanding Volume in a MongoDB Cluster

This guide explains how to expand Persistent Volume Claims (PVCs) in a MongoDB cluster managed by KubeBlocks. Volume expansion enables dynamic storage capacity increases, allowing your database to scale seamlessly as data grows. When supported by the underlying storage class, this operation can be performed without downtime.

Volume expansion allows you to increase the size of a Persistent Volume Claim (PVC) after it has been created. This feature was introduced in Kubernetes v1.11 and became generally available (GA) in Kubernetes v1.24.

Prerequisites

Before proceeding, ensure the following:

• Environment Setup:
  • A Kubernetes cluster is up and running.
  • The kubectl CLI tool is configured to communicate with your cluster.
  • KubeBlocks CLI and KubeBlocks Operator are installed. Follow the installation instructions here.
• Namespace Preparation: To keep resources isolated, create a dedicated namespace for this tutorial:

kubectl create ns demo
namespace/demo created

Check the Storage Class for Volume Expansion Support

List all available storage classes and verify if volume expansion is supported by checking the ALLOWVOLUMEEXPANSION field:

kubectl get storageclass

Example Output:

NAME                PROVISIONER             RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
gp2                 kubernetes.io/aws-ebs   Delete          WaitForFirstConsumer   false                  4d10h
kb-default-sc       ebs.csi.aws.com         Delete          WaitForFirstConsumer   true                   3d7h
sc-s3-repo-2qsxfh   ru.yandex.s3.csi        Retain          Immediate              false                  3d7h

Ensure the storage class you are using has ALLOWVOLUMEEXPANSION set to true. If it is false, the storage class does not support volume expansion.

Deploy a MongoDB ReplicaSet Cluster with StorageClass

KubeBlocks uses a declarative approach to manage MongoDB clusters. Below is an example configuration for deploying a MongoDB cluster with 2 replicas (1 primary, 1 secondary).

Apply the following YAML configuration to deploy the cluster:

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: mongo-cluster
  namespace: demo
spec:
  terminationPolicy: Delete
  clusterDef: mongodb
  topology: replicaset
  componentSpecs:
    - name: mongodb
      serviceVersion: "6.0.21"
      replicas: 3
      resources:
        limits:
          cpu: '0.5'
          memory: 0.5Gi
        requests:
          cpu: '0.5'
          memory: 0.5Gi
      volumeClaimTemplates:
        - name: data
          spec:
            # specify storage class name supports Volume Expansion
            storageClassName: <STORAGE_CLASS_NAME>
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 20Gi

Explanation of Key Fields

• storageClassName: Specifies StorageClass name that supports volume expansion. If not set, the StorageClass annotated default will be used.

Verifying the Deployment

Monitor the cluster status until it transitions to the Running state:

kubectl get cluster mongo-cluster -n demo -w

Expected Output:

kubectl get cluster mongo-cluster -n demo
NAME            CLUSTER-DEFINITION   TERMINATION-POLICY   STATUS     AGE
mongo-cluster   mongodb              Delete               Creating   49s
mongo-cluster   mongodb              Delete               Running    62s

Check the pod status and roles:

kubectl get pods -l app.kubernetes.io/instance=mongo-cluster -L  kubeblocks.io/role -n demo

Expected Output:

NAME                      READY   STATUS    RESTARTS   AGE   ROLE
mongo-cluster-mongodb-0   2/2     Running   0          78s   primary
mongo-cluster-mongodb-1   2/2     Running   0          63s   secondary
mongo-cluster-mongodb-2   2/2     Running   0          48s   secondary

Once the cluster status becomes Running, your MongoDB cluster is ready for use.

Expand volume

You can expand the volume in one of two ways:

apiVersion: operations.kubeblocks.io/v1alpha1
kind: OpsRequest
metadata:
  name: mongo-cluster-expand-volume-ops
  namespace: demo
spec:
  clusterName: mongo-cluster
  type: VolumeExpansion
  volumeExpansion:
  - componentName: mongodb
    volumeClaimTemplates:
    - name: data
      storage: 30Gi

kubectl describe ops mongo-cluster-expand-volume-ops -n demo

Status:
  Phase:            Succeed

componentSpecs:
  - name: mongodb
    volumeClaimTemplates:
      - name: data
        spec:
          storageClassName: <STORAGE_CLASS_NAME>
          accessModes:
            - ReadWriteOnce
          resources:
            requests:
              # specify new size, and make sure it is larger than current size
              storage: 30Gi

Verification

Verify the updated cluster configuration:

kbcli cluster describe mongo-cluster -n demo

Expected Output:

Resources Allocation:
COMPONENT   INSTANCE-TEMPLATE     CPU(REQUEST/LIMIT)   MEMORY(REQUEST/LIMIT)   STORAGE-SIZE   STORAGE-CLASS
mongodb                           500m / 500m          512Mi / 512Mi           data:30Gi      <STORAGE_CLASS_NAME>

The volume size for the data PVC has been updated to the specified value (e.g., 30Gi in this case).

Confirm PVC resizing completion:

kubectl get pvc -l app.kubernetes.io/instance=mongo-cluster -n demo

Expected Output:

NAME                           STATUS   VOLUME    CAPACITY   ACCESS MODES   STORAGECLASS          AGE
mongo-cluster-mongodb-data-0   Bound    pvc-uuid  30Gi       RWO            <STORAGE_CLASS_NAME>  33m
mongo-cluster-mongodb-data-1   Bound    pvc-uuid  30Gi       RWO            <STORAGE_CLASS_NAME>  33m
mongo-cluster-mongodb-data-2   Bound    pvc-uuid  30Gi       RWO            <STORAGE_CLASS_NAME>  33m

Cleanup

To remove all created resources, delete the MongoDB cluster along with its namespace:

kubectl delete cluster mongo-cluster -n demo
kubectl delete ns demo

Summary

In this guide you learned how to:

1. Verify storage class compatibility for volume expansion.
2. Perform volume expansion using either:
   • OpsRequest for dynamic updates.
   • Cluster API for manual updates.
3. Verify the updated PVC size and ensure the resize operation is complete.

With volume expansion, you can efficiently scale your MongoDB cluster's storage capacity without service interruptions, ensuring your database can grow alongside your application needs.