fix(docs): Update migration.md with LDAP PVC update information.

docs/migrations.md

@@ -8,6 +8,7 @@ SPDX-License-Identifier: Apache-2.0
 * [Disclaimer](#disclaimer)
 * [From v0.8.1](#from-v081)
   * [Updated `cluster.networking.cidr`](#updated-clusternetworkingcidr)
+  * [Nubus LDAP PVCs](#nubus-ldap-pvcs)
   * [Updated customizable template attributes](#updated-customizable-template-attributes)
   * [`migrations` S3 bucket](#migrations-s3-bucket)
 
@@ -24,6 +25,60 @@ Though we try to ease the pain when it comes to 0.x upgrades. That is what this
 - Action: `cluster.networking.cidr` is now an array (was a string until 0.8.1), please update your setup accordingly if you explicitly set this value.
 - Reference:[cluster.yaml](https://gitlab.opencode.de/bmi/opendesk/deployment/opendesk/-/blob/main/helmfile/environments/default/cluster.yaml)
 
+## Nubus LDAP PVCs
+
+openDesk is integrating the latest [Nubus](https://www.univention.de/produkte/nubus/) development from Univention. The new redundant and scalable LDAP requires some manual action to upgrade from 0.8.1:
+
+- Action: Before the upgrade you have to prepare the PVCs for the LDAP primary Pods. First scale down the 0.8.1 LDAP Pod and pre-create and pre-populate the new PVCs with the data from the current LDAP PVC. You can do all this by running the following snippet on your commandline, after setting `NAMESPACE` to the appropriate value. The LDAP secondaries get sync'd from the primary to fill their own PVCs data.
+```
+export NAMESPACE=YOUR_NAMESPACE
+kubectl -n $NAMESPACE scale --replicas=0 statefulset/ums-ldap-notifier
+kubectl -n $NAMESPACE scale --replicas=0 statefulset/ums-ldap-server
+kubectl -n $NAMESPACE apply -f - <<EOF
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  # Target PVC name
+  name: shared-data-ums-ldap-server-primary-0
+spec:
+  dataSource:
+    # Source PVC name
+    name: shared-data-ums-ldap-server-0
+    kind: PersistentVolumeClaim
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      # Target PVC size (deployments default to 1Gi)
+      storage: 1Gi
+...
+---
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  # Target PVC name
+  name: shared-data-ums-ldap-server-primary-1
+spec:
+  dataSource:
+    # Source PVC name
+    name: shared-data-ums-ldap-server-0
+    kind: PersistentVolumeClaim
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      # Target PVC size (deployments default to 1Gi)
+      storage: 1Gi
+...
+EOF
+```
+
+- Once you have verified that your upgrade was successful, you can delete the previous LDAP's PVC:
+```
+kubectl -n $NAMESPACE delete pvc shared-data-ums-ldap-server-0
+```
+
 ## Updated customizable template attributes
 
 - Action: Please ensure you update you custom deployment values according with the updated default value structure.