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MongoDB Replicaset Volume Expansion

This guide will show you how to use KubeDB Enterprise operator to expand the volume of a MongoDB Replicaset database.

Before You Begin

  • At first, you need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster.

  • You must have a StorageClass that supports volume expansion.

  • Install KubeDB Community and Enterprise operator in your cluster following the steps here.

  • You should be familiar with the following KubeDB concepts:

To keep everything isolated, we are going to use a separate namespace called demo throughout this tutorial.

$ kubectl create ns demo
namespace/demo created

Note: YAML files used in this tutorial are stored in docs/examples/mongodb directory of kubedb/docs repository.

Expand Volume of Replicaset

Here, we are going to deploy a MongoDB replicaset using a supported version by KubeDB operator. Then we are going to apply MongoDBOpsRequest to expand its volume.

Prepare MongoDB Replicaset Database

At first verify that your cluster has a storage class, that supports volume expansion. Let’s check,

$ kubectl get storageclass                                                                                                                                           20:22:33
NAME                 PROVISIONER            RECLAIMPOLICY   VOLUMEBINDINGMODE   ALLOWVOLUMEEXPANSION   AGE
standard (default)   kubernetes.io/gce-pd   Delete          Immediate           true                   2m49s

We can see from the output the standard storage class has ALLOWVOLUMEEXPANSION field as true. So, this storage class supports volume expansion. We can use it.

Now, we are going to deploy a MongoDB replicaSet database with version 3.6.8.

Deploy MongoDB

In this section, we are going to deploy a MongoDB Replicaset database with 1GB volume. Then, in the next section we will expand its volume to 2GB using MongoDBOpsRequest CRD. Below is the YAML of the MongoDB CR that we are going to create,

apiVersion: kubedb.com/v1alpha2
kind: MongoDB
metadata:
  name: mg-replicaset
  namespace: demo
spec:
  version: "3.6.8-v1"
  replicaSet: 
    name: "replicaset"
  replicas: 3
  storageType: Durable
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 1Gi

Let’s create the MongoDB CR we have shown above,

$ kubectl create -f https://github.com/kubedb/docs/raw/v2020.11.12/docs/examples/mongodb/volume-expansion/mg-replicaset.yaml
mongodb.kubedb.com/mg-replicaset created

Now, wait until mg-replicaset has status Running. i.e,

$ kubectl get mg -n demo                                                                                                                                             20:05:47
NAME            VERSION    STATUS    AGE
mg-replicaset   3.6.8-v1   Running   10m

Let’s check volume size from statefulset, and from the persistent volume,

$ kubectl get sts -n demo mg-replicaset -o json | jq '.spec.volumeClaimTemplates[].spec.resources.requests.storage'
"1Gi"

$ kubectl get pv -n demo                                                                                          
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                          STORAGECLASS   REASON   AGE
pvc-2067c63d-f982-4b66-a008-5e9c3ff6218a   1Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-0   standard                10m
pvc-9db1aeb0-f1af-4555-93a3-0ca754327751   1Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-2   standard                9m45s
pvc-d38f42a8-50d4-4fa9-82ba-69fc7a464ff4   1Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-1   standard                10m

You can see the statefulset has 1GB storage, and the capacity of all the persistent volumes are also 1GB.

We are now ready to apply the MongoDBOpsRequest CR to expand the volume of this database.

Volume Expansion

Here, we are going to expand the volume of the replicaset database.

Create MongoDBOpsRequest

In order to expand the volume of the database, we have to create a MongoDBOpsRequest CR with our desired volume size. Below is the YAML of the MongoDBOpsRequest CR that we are going to create,

apiVersion: ops.kubedb.com/v1alpha1
kind: MongoDBOpsRequest
metadata:
  name: mops-volume-exp-replicaset
  namespace: demo
spec:
  type: VolumeExpansion  
  databaseRef:
    name: mg-replicaset
  volumeExpansion:
    replicaSet: 2Gi

Here,

  • spec.databaseRef.name specifies that we are performing volume expansion operation on mops-volume-exp-replicaset database.
  • spec.type specifies that we are performing VolumeExpansion on our database.
  • spec.volumeExpansion.replicaSet specifies the desired volume size.

Let’s create the MongoDBOpsRequest CR we have shown above,

$ kubectl apply -f https://github.com/kubedb/docs/raw/v2020.11.12/docs/examples/mongodb/volume-expansion/mops-volume-exp-replicaset.yaml
mongodbopsrequest.ops.kubedb.com/mops-volume-exp-replicaset created

Verify MongoDB replicaset volume expanded successfully

If everything goes well, KubeDB Enterprise operator will update the volume size of MongoDB object and related StatefulSets and Persistent Volumes.

Let’s wait for MongoDBOpsRequest to be Successful. Run the following command to watch MongoDBOpsRequest CR,

$ kubectl get mongodbopsrequest -n demo
Every 2.0s: kubectl get mongodbopsrequest -n demo
NAME                         TYPE              STATUS       AGE
mops-volume-exp-replicaset   VolumeExpansion   Successful   83s

We can see from the above output that the MongoDBOpsRequest has succeeded. If we describe the MongoDBOpsRequest we will get an overview of the steps that were followed to expand the volume of the database.

$ kubectl describe mongodbopsrequest -n demo mops-volume-exp-replicaset   
Name:         mops-volume-exp-replicaset
Namespace:    demo
Labels:       <none>
Annotations:  API Version:  ops.kubedb.com/v1alpha1
Kind:         MongoDBOpsRequest
Metadata:
  Creation Timestamp:  2020-08-25T18:21:18Z
  Finalizers:
    kubedb.com
  Generation:        1
  Resource Version:  84084
  Self Link:         /apis/ops.kubedb.com/v1alpha1/namespaces/demo/mongodbopsrequests/mops-volume-exp-replicaset
  UID:               2cec0cd3-4abe-4114-813c-1326f28563cb
Spec:
  Database Ref:
    Name:  mg-replicaset
  Type:    VolumeExpansion
  Volume Expansion:
    ReplicaSet:  2Gi
Status:
  Conditions:
    Last Transition Time:  2020-08-25T18:21:18Z
    Message:               MongoDB ops request is being processed
    Observed Generation:   1
    Reason:                Scaling
    Status:                True
    Type:                  Scaling
    Last Transition Time:  2020-08-25T18:22:38Z
    Message:               Successfully updated Storage
    Observed Generation:   1
    Reason:                VolumeExpansion
    Status:                True
    Type:                  VolumeExpansion
    Last Transition Time:  2020-08-25T18:22:38Z
    Message:               Successfully Resumed mongodb: mg-replicaset
    Observed Generation:   1
    Reason:                ResumeDatabase
    Status:                True
    Type:                  ResumeDatabase
    Last Transition Time:  2020-08-25T18:22:38Z
    Message:               Successfully completed the modification process
    Observed Generation:   1
    Reason:                Successful
    Status:                True
    Type:                  Successful
  Observed Generation:     1
  Phase:                   Successful
Events:
  Type    Reason           Age    From                        Message
  ----    ------           ----   ----                        -------
  Normal  VolumeExpansion  3m11s  KubeDB Enterprise Operator  Successfully Updated Storage
  Normal  ResumeDatabase   3m11s  KubeDB Enterprise Operator  Resuming MongoDB
  Normal  ResumeDatabase   3m11s  KubeDB Enterprise Operator  Successfully Resumed mongodb
  Normal  Successful       3m11s  KubeDB Enterprise Operator  Successfully Scaled Database  

Now, we are going to verify from the Statefulset, and the Persistent Volumes whether the volume of the database has expanded to meet the desired state, Let’s check,

$ kubectl get sts -n demo mg-replicaset -o json | jq '.spec.volumeClaimTemplates[].spec.resources.requests.storage'
"2Gi"

$ kubectl get pv -n demo                                                                                          
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                          STORAGECLASS   REASON   AGE
pvc-2067c63d-f982-4b66-a008-5e9c3ff6218a   2Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-0   standard                19m
pvc-9db1aeb0-f1af-4555-93a3-0ca754327751   2Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-2   standard                18m
pvc-d38f42a8-50d4-4fa9-82ba-69fc7a464ff4   2Gi        RWO            Delete           Bound    demo/datadir-mg-replicaset-1   standard                19m

The above output verifies that we have successfully expanded the volume of the MongoDB database.

Cleaning Up

To clean up the Kubernetes resources created by this tutorial, run:

kubectl delete mg -n demo mg-replicaset
kubectl delete mongodbopsrequest -n demo mops-volume-exp-replicaset