On Thu, Aug 2, 2012 at 8:09 AM, Dong Xu Wang <wdon...@linux.vnet.ibm.com> wrote: > On Wed, Aug 1, 2012 at 9:55 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> On Tue, Jul 31, 2012 at 5:51 PM, Dong Xu Wang >> <wdon...@linux.vnet.ibm.com> wrote: >>> +test.raw may be larger than ubuntu.img, in that case, the size of >>> test.add-cow >>> +will be calculated by the size of ubuntu.img, test.raw will be used from >>> the >>> +1st byte, the rest part can be used for other purpose. >> >> This is not how backing files normally work. With qcow2 or qed a >> smaller backing file just means that the guest reads zeroes from the >> areas beyond the end of the backing file. Is there a special reason >> why you want to implement the behavior you described in the spec? >> Otherwise I suggest implementing the same behavior as qcow2/qed. > > The size of add-cow will be caclulated by the size of backing_file, if > backing_file can be smaller, how can I get the size of add-cow while > being created? > > Do you mean like following steps? > 1) qemu-img create -f qcow2 source.qcow2 *8G* > 2) qemu-img create -f t.add-cow -o backing_file=source.qcow2,image_file=t > *10G* > And then reading un-allocated bytes from add-cow after 8G will be 0?
Yes. You could also get the virtual disk size from the size of the image_file during creation. Stefan
