Add longhorn trim info (#30402)

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website/src/content/docs/clustertool/csi/longhorn.md

@@ -19,6 +19,31 @@ Their Helm-Chart is available at: <https://github.com/longhorn/charts>
 
 Longhorn is included with ClusterTool. It rolls out a, default, storageClass named longhorn, that saves its data on the systemdisk.
 
+## Issues with Longhorn and Volsync
+
+Volsync creates backup each day and uses snapshots to do so. This leads to a lot of snapshot creations which will be deleted after the backup is done.
+
+Longhorn wont free/trim the space gained by the deletion of the snapshots and will keep it as used. This can lead to a no-disk-space error after some time depending on the amount of volsync backups and their size.
+This can be prevented by adding the following `ReccuringTask` to Longhorn:
+
+```yaml
+
+apiVersion: longhorn.io/v1beta2
+kind: RecurringJob
+metadata:
+  name: trim
+  namespace: longhorn-system
+spec:
+  concurrency: 1
+  cron: 0 0 * * *
+  groups:
+  - default
+  name: trim
+  retain: 0
+  task: filesystem-trim
+
+```
+This runs a filesystem trim on midnight cet. To free the "used" space. 
 
 ## Other references