No, it's 3-Par. But I think I found the cause. The customer is using an NFS-share on Windows 2012R2 for synchronisation between datastores/datacenters. I think Windows 2012R2 NFS is not a supported storage for vSphere. Only strange thing is that initially the AppStack is thin provisioned. If I update the Appstack before replication has occurred, a thin provisioned clone is created. But if I update the Appstack after it has been replicated to the NFS-share and from the NFS-share to the other datastore, the copied AppStack also becomes thick provisioned.