Thanks, Oracle VM is based on XEN, I will try the qemu-img command.
I want to leave Oracle VM becaue their management console should run on a standalone Linux machine.
About a year ago before I use Oracle VM, I tried Proxmox, but at that time Proxmox does not support external stroage, now it supports iSCSI I think should try it again. And I like Proxmox web admin more than Oracle VM.