On Thu, May 17, 2012 at 02:52:56PM +0100, 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...
Well I'll say first of all that I was asked to implement this by a colleague. Personally, I'm far more organized than this, and I always use snapshots and backing files if I want to create an efficient COW from a base template :-) However my colleague has got himself into a situation where he has copied (ie. "cp" or equivalent) a guest several times from a template, and these guests have been running independently. He now wants to conserve disk space by turning this situation back into one where he has one backing file + several COW copies. To do this, he can (with this patch) do: qemu-img diff -b base.img the_copied_guest.img guest.qcow2 rm the_copied_guest.img 'guest.qcow2' will (in theory at least) be much smaller than the copied guests he has right now. Does that make sense? [BTW I'm still working on this. There are a few spelling mistakes and it needs a lot more testing. This patch is just for comment at the moment.] Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
