I noticed that cow adds space also if the written data is the same as the one it was there already. Is there a reason why checking it would be bad/difficult/slow/other? Simply no one had the will to code the check?
I ask because for shrinking drives I usually create big zero-filled files... and this grows the qcow2 image like mad, and only a compressed convert can properly shrink it back. OK, it is not to be done very very often, but not copying those writes that don't actually change the data seems like a good idea... don't know about the speed hit, though. Another idea would be to detect "only zero" write blocks and simply interpret them as "put a hole in the sparse file" instead of actually writing data to disk. Just the first idea that hit me after a few hours of qemu usage, don't hit me with a brick if it has already been discussed to death: I searched it in the gmane.org archives to no avail ;-) -- Lapo Luchini [EMAIL PROTECTED] (OpenPGP & X.509) www.lapo.it (Jabber, ICQ, MSN)
