On 05/17/2012 07:52 AM, Peter Maydell wrote: > On 17 May 2012 14:44, Richard W.M. Jones <rjo...@redhat.com> wrote: >> From: "Richard W.M. Jones" <rjo...@redhat.com> >> >> This produces a qcow2 file which is the different between >> two disk images. ie, if: >> >> original.img - is a disk image (in any format) >> modified.img - is a modified version of original.img >> >> then: >> >> qemu-img diff -b original.img modified.img diff.qcow2 >> >> creates 'diff.qcow2' which contains just the differences. Note that >> 'diff.qcow2' has 'original.img' set as the backing file. > > Any chance of some more detailed explanation in the docs patch > about what this actually means and why it's useful? I spent > several minutes going "huh, does it even mean anything to > calculate the difference between two binary disk images?" > before realising that it's the presence of the backing file > that makes it actually make sense...
Even something as simple as: Useful for converting a monolithic image back into a thin image on top of a common base. -- Eric Blake ebl...@redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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