I think the op is meaning, like when you download a game, steam reserves the entire 100G of space for the game download right off, as should creating a vdisk.... and yes, it is possible for qemu to do this without taking forever writing a huge file... virt-manger does this by default. this behavior helps prevent mistakes, later on when creating multiple vms, or multiple disk images, and then you forget how much space you really have left on the physical device. In unraids annoying behavior, I am usually left to fill each disk image up with a temp file myself, usually a bunch of movies from my plex server