# High availability volume

HwameiStor uses the open source DRBD data synchronization technology to create **Highly Available Volume**, also known as **HA Volume**.

Here we use a MySQL application as an example.

!!! note

    The MySQL Yaml file below is from [Kubernetes official repo](https://github.com/kubernetes/website/blob/main/content/en/examples/application/mysql/mysql-statefulset.yaml)

## View `StorageClass`

`StorageClass` "hwameistor-storage-lvm-hdd-ha" uses the parameter `replicaNumber: "2"` to enable high availability:

```console
$ kubectl apply -f examples/sc_ha.yaml
$ kubectl get sc hwameistor-storage-lvm-hdd-ha -o yaml

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: hwameistor-storage-lvm-hdd-ha
parameters:
  replicaNumber: "2"
  convertible: "false"
  csi.storage.k8s.io/fstype:xfs
  poolClass: HDD
  poolType: REGULAR
  striped: "true"
  volumeKind: LVM
provisioner: lvm.hwameistor.io
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true
```

## Create `StatefulSet`

After HwameiStor and `StorageClass` are ready, one command can create MySQL container and its data volume:

```sh
kubectl apply -f exapmles/sts-mysql_ha.yaml
```

Note that `volumeClaimTemplates` uses `storageClassName: hwameistor-storage-lvm-hdd-ha`:

```yaml
spec:
  volumeClaimTemplates:
  - metadata:
      name: data
      labels:
        app: sts-mysql-ha
        app.kubernetes.io/name: sts-mysql-ha
    spec:
      storageClassName: hwameistor-storage-lvm-hdd-ha
      accessModes: ["ReadWriteOnce"]
      resources:
        requests:
          storage: 1Gi
```

## View MySQL Pods and `PVC/PV`

In this example, the MySQL container is scheduled to node `k8s-worker-3`.

```console
$ kubectl get po -l app=sts-mysql-ha -o wide
NAME READY STATUS RESTARTS AGE IP NODE
sts-mysql-ha-0 2/2 Running 0 3m08s 10.1.15.151 k8s-worker-1

$ kubectl get pvc -l app=sts-mysql-ha
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGE CLASS AGE VOLUME MODES
data-sts-mysql-ha-0 Bound pvc-5236ee6f-8212-4628-9876-1b620a4c4c36 1Gi RWO hwameistor-storage-lvm-hdd 3m Filesystem
```

## View `LocalVolume` and `LocalVolumeReplica` objects

By viewing `LocalVolume(LV)` with the same name as `PV`, you can see that the local volume is created on node `k8s-worker-1` and node `k8s-worker-2`.

```console
$ kubectl get lv pvc-5236ee6f-8212-4628-9876-1b620a4c4c36

NAME POOL REPLICAS CAPACITY ACCESSIBILITY STATE RESOURCE PUBLISHED AGE
pvc-5236ee6f-8212-4628-9876-1b620a4c4c36 LocalStorage_PoolHDD 1 1073741824 Ready-1 k8s-worker-1,k8s-worker-2 3m
```

`LocalVolumeReplica (LVR)` further shows the backend logical volume devices on each node:

```console
kubectl get lvr
NAME CAPACITY NODE STATE SYNCED DEVICE AGE
5236ee6f-8212-4628-9876-1b620a4c4c36-d2kn55 1073741824 k8s-worker-1 Ready true /dev/LocalStorage_PoolHDD-HA/5236ee6f-8212-4628-9876-1b620a4c4c36 4m
5236ee6f-8212-4628-9876-1b620a4c4c36-glm7rf 1073741824 k8s-worker-3 Ready true /dev/LocalStorage_PoolHDD-HA/5236ee6f-8212-4628-9876-1b620a4c4c36 4m
```