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OpenEBS cStor walkthrough

After creating the cStor disk pool, our logical next step is to create a storage class which uses this specific disk pool.

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: cstor-csi-disk
provisioner: cstor.csi.openebs.io
allowVolumeExpansion: true
parameters:
  cas-type: cstor
  cstorPoolCluster: cstor-disk-pool
  replicaCount: "3"

We just created a storage class for cStor, which leverages the cstor-disk-pool we created, with a replica count of 3.

Putting the cStor storage class to use #

To illustrate other concepts like volume snapshotting and volume cloning, we shall deploy a MySQL database in our cluster. We will use the Bitnami MySQL chart for this. Here’s how the helm values will look like:

global:
  storageClass: cstor-csi-disk
auth:
  rootPassword: "root123"
primary:
  persistence:
    size: "4Gi"

Let’s deploy the chart in the default namespace.

$ helm repo add bitnami https://charts.bitnami.com/bitnami
$ helm repo update
$ helm install my-db bitnami/mysql --values mysql-values.yml

We can observe that after a minute or so, the following artefacts get created:

  1. A MySQL statefulset/pod
  2. A corresponding PVC
  3. A cStor storage controller in openebs namespace with 3 replicas

Now, we will login to the MySQL pod and add some data.

$ kubectl exec -it my-db-mysql-0 -- bash
# inside the pod
I have no name!@my-db-mysql-0:/$ mysql -u root -p"$MYSQL_ROOT_PASSWORD" -D my_database

Run these queries which will create an employees table and add 10 rows. I got this sample data from Mockaroo.

create table employees (
	id INT,
	first_name VARCHAR(50),
	last_name VARCHAR(50),
	department VARCHAR(50),
	gender VARCHAR(50)
);
insert into employees (id, first_name, last_name, department, gender) values (1, 'Blair', 'Nowick', 'Accounting', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (2, 'Ralf', 'MacKenney', 'Sales', 'Male');
insert into employees (id, first_name, last_name, department, gender) values (3, 'Kordula', 'Haggath', 'Training', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (4, 'Nancey', 'Lamburn', 'Research and Development', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (5, 'Christoffer', 'Danielut', 'Engineering', 'Male');
insert into employees (id, first_name, last_name, department, gender) values (6, 'Wynn', 'Gable', 'Product Management', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (7, 'Evaleen', 'Ahren', 'Accounting', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (8, 'Farrah', 'Haggart', 'Engineering', 'Female');
insert into employees (id, first_name, last_name, department, gender) values (9, 'Moshe', 'McCarter', 'Sales', 'Genderqueer');
insert into employees (id, first_name, last_name, department, gender) values (10, 'Ferd', 'Boulsher', 'Research and Development', 'Male');

Snapshotting volumes #

After adding 10 rows, let’s take a snapshot of the current volume. I’ll have to mention that the volume snapshotting is a general Kubernetes feature and not specific to any storage class or provider. Most providers support this since 1.20. We will scope this article to cStor. Similar to how there is a storage class, Persistent volume claim and Persistent volume, volume snapshot also has equivalent API resources. This is best represented by an image.

Persistent volumes and snapshot API resources comparison
Persistent volumes and snapshot API resources comparison

A VolumeSnapshotClass describes the storage policy, storage driver used etc. for any volume snapshot which gets provisioned through this snapshot class.

kind: VolumeSnapshotClass
apiVersion: snapshot.storage.k8s.io/v1
metadata:
  name: csi-cstor-snapshotclass
  annotations:
    snapshot.storage.kubernetes.io/is-default-class: "true"
driver: cstor.csi.openebs.io
deletionPolicy: Delete

Now, we create a VolumeSnapshot using this snapshot class. Here, we specify what PVC we have to snapshot.

apiVersion: snapshot.storage.k8s.io/v1
kind: VolumeSnapshot
metadata:
  name: data-my-db-mysql-0-snapshot
spec:
  volumeSnapshotClassName: csi-cstor-snapshotclass
  source:
    persistentVolumeClaimName: data-my-db-mysql-0

When we apply this, we will see 2 artefacts getting created.

  1. A volume snapshot.
  2. A volume snapshot content.
$ kubectl get volumesnapshot
NAME                          READYTOUSE   SOURCEPVC            SOURCESNAPSHOTCONTENT   RESTORESIZE   SNAPSHOTCLASS             SNAPSHOTCONTENT                                    CREATIONTIME   AGE
data-my-db-mysql-0-snapshot   true         data-my-db-mysql-0                           0             csi-cstor-snapshotclass   snapcontent-ec920b9c-d73f-484b-88e9-53ee6374830d   12h            12h

$ kubectl get volumesnapshotcontent
NAME                                               READYTOUSE   RESTORESIZE   DELETIONPOLICY   DRIVER                 VOLUMESNAPSHOTCLASS       VOLUMESNAPSHOT                AGE
snapcontent-ec920b9c-d73f-484b-88e9-53ee6374830d   true         0             Delete           cstor.csi.openebs.io   csi-cstor-snapshotclass   data-my-db-mysql-0-snapshot   12h

Cloning a volume snapshot #

The snapshot contains “point-in-time” copy of the volume we snapshotted. This means that any data added to the volume after taking a snapshot won’t get persisted in the snapshot. To demonstrate this fact, let’s get into the pod, login to MySQL prompt and add 2 more rows.

insert into employees (id, first_name, last_name, department, gender) values (11, 'Kelvin', 'Kohnen', 'Legal', 'Male');
insert into employees (id, first_name, last_name, department, gender) values (12, 'Millie', 'Kingswood', 'Research and Development', 'Female');

Now, the employees table contains 12 rows. We will create a new PVC out of this snapshot.

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apiVersion: v1
kind: PersistentVolumeClaim
metadata:
 name: restore-mysql-pvc
spec:
 storageClassName: cstor-csi-disk
 dataSource:
   name: data-my-db-mysql-0-snapshot
   kind: VolumeSnapshot
   apiGroup: snapshot.storage.k8s.io
 accessModes:
   - ReadWriteOnce
 resources:
   requests:
     storage: 4Gi

Notice the dataSource part highlighted. It indicates that we create this PVC out of a volume snapshot. After this PVC is created, we will now deploy a new MySQL installation which utilizes this PVC.

$ helm install my-db-clone bitnami/mysql --values mysql-values-from-clone.yml

Here’s the helm values file referenced above.

global:
  storageClass: cstor-csi-disk
auth:
  rootPassword: "root123"
primary:
  persistence:
    existingClaim: "restore-mysql-pvc"

Now, when we login to the newly created MySQL installation which references the cloned PVC,

kubectl exec -it my-db-clone-mysql-0 -- bash

We notice that it contains 10 employees.

$ kubectl exec -it my-db-clone-mysql-0 -- bash
I have no name!@my-db-clone-mysql-0:/$ mysql -u root -p"$MYSQL_ROOT_PASSWORD" -D my_database
mysql: [Warning] Using a password on the command line interface can be insecure.
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 9241
Server version: 8.0.31 Source distribution

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select count(*) from employees;
+----------+
| count(*) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)

mysql>

This need not be done manually and can be scheduled as a backup and restore process, as we will see in the next post.

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