That's true -- I forgot about that. The one caveat to that is that it is primarily targeted by the XenServer product: Xen, KVM, and QEMU tend to use the qcow or raw formats. I'm also worried about new features coming out from one or the other VMMs that aren't supported by the current format. Without an established way for anyone to adapt the format, I'm not sure it will provide a long term solution. I do have a specific feature in mind, but unfortunately you will have to wait for the colleague's PhD thesis to find out about it ;)