Add docs

Signed-off-by: d-kuro <kurosawa7620@gmail.com>

docs/change-pvc-template.md

@@ -62,6 +62,99 @@ so users can notice anomalies by monitoring this metrics.
 
 See the [metrics documentation](./metrics.md) for more details.
 
+## Volume reduction
+
+MOCO supports PVC reduction, but unlike PVC expansion, the user must perform the operation manually.
+
+The steps are as follows:
+
+1. The user modifies the `.spec.volumeClaimTemplates` of the MySQLCluster and sets a smaller volume size.
+2. MOCO updates the `.spec.volumeClaimTemplates` of the StatefulSet. This does not propagate to existing Pods, PVCs, or PVs.
+3. The user manually deletes the MySQL Pod & PVC.
+4. Wait for the Pod & PVC to be recreated by the statefulset-controller, and for MOCO to clone the data.
+5. Once the cluster becomes Healthy, the user deletes the next Pod and PVC.
+6. It is completed when all Pods and PVCs are recreated.
+
+### 1. The user modifies the `.spec.volumeClaimTemplates` of the MySQLCluster and sets a smaller volume size
+
+For example, the user modifies the `.spec.volumeClaimTemplates` of the MySQLCluster as follows:
+
+```diff
+  apiVersion: moco.cybozu.com/v1beta2
+  kind: MySQLCluster
+  metadata:
+    namespace: default
+    name: test
+  spec:
+    replicas: 3
+    podTemplate:
+      spec:
+        containers:
+        - name: mysqld
+          image: quay.io/cybozu/mysql:8.0.30
+    volumeClaimTemplates:
+    - metadata:
+        name: mysql-data
+      spec:
+        accessModes: [ "ReadWriteOnce" ]
+        resources:
+          requests:
+-           storage: 1Gi
++           storage: 500Mi
+```
+
+### 2. MOCO updates the `.spec.volumeClaimTemplates` of the StatefulSet. This does not propagate to existing Pods, PVCs, or PVs
+
+The moco-controller will update the `.spec.volumeClaimTemplates` of the StatefulSet.
+The actual modification of the StatefulSet's `.spec.volumeClaimTemplates` is not allowed,
+so this change is achieved by recreating the StatefulSet.
+At this time, only the recreation of StatefulSet is performed, without deleting the Pods and PVCs.
+
+### 3. The user manually deletes the MySQL Pod & PVC
+
+The user manually deletes the PVC and Pod.
+Use the following command to delete them:
+
+```console
+$ kubectl delete --wait=false pvc <pvc-name>
+$ kubectl delete --grace-period=1 <pod-name>
+```
+
+### 4. Wait for the Pod & PVC to be recreated by the statefulset-controller, and for MOCO to clone the data
+
+The statefulset-controller recreates Pods and PVCs, creating a new PVC with a reduced size.
+Once the MOCO successfully starts a Pod, it begins cloning the data.
+
+```console
+$ kubectl get mysqlcluster,po,pvc
+NAME                                AVAILABLE   HEALTHY   PRIMARY   SYNCED REPLICAS   ERRANT REPLICAS   LAST BACKUP
+mysqlcluster.moco.cybozu.com/test   True        False     0         2                                   <no value>
+
+NAME              READY   STATUS     RESTARTS   AGE
+pod/moco-test-0   3/3     Running    0          2m14s
+pod/moco-test-1   3/3     Running    0          114s
+pod/moco-test-2   0/3     Init:1/2   0          7s
+
+NAME                                           STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
+persistentvolumeclaim/mysql-data-moco-test-0   Bound    pvc-03c73525-0d6d-49de-b68a-f8af4c4c7faa   1Gi        RWO            standard       2m14s
+persistentvolumeclaim/mysql-data-moco-test-1   Bound    pvc-73c26baa-3432-4c85-b5b6-875ffd2456d9   1Gi        RWO            standard       114s
+persistentvolumeclaim/mysql-data-moco-test-2   Bound    pvc-779b5b3c-3efc-4048-a549-a4bd2d74ed4e   500Mi      RWO            standard       7s
+```
+
+### 5. Once the cluster becomes Healthy, the user deletes the next Pod and PVC
+
+The user waits until the MySQLCluster state becomes Healthy, and then deletes the next Pod and PVC.
+
+```console
+$ kubectl get mysqlcluster
+NAME                                AVAILABLE   HEALTHY   PRIMARY   SYNCED REPLICAS   ERRANT REPLICAS   LAST BACKUP
+mysqlcluster.moco.cybozu.com/test   True        True      1         3                                   <no value>
+```
+
+### 6. It is completed when all Pods and PVCs are recreated
+
+Repeat steps 3 to 5 until all Pods and PVCs are recreated.
+
 ## References
 
 * [Design document](./designdoc/support_apply_pvc_template_changes.md)