Related to storage that support snapshot, digging around I've found that recent LVM support the "lvconvert –merge". As far as I know, has become available since kernel 2.6.33 (and the lvm parts from 2.02.58).
Is something that would work as well as qcow2, once Proxmox will go to a more recent kernel or this feature will be backoporte (maybe by Red Hat?).
Just curious, because I tend to use LVM storage for performance reasons, qcow2 is known to have not stunning I/O performance, and the "snapshot" feature is really valuable when you have to test updates or something like that.