$ qemu-img create -f qcow2 -b /tmp/centos-6.img overlay.qcow2 $ virt-sparsify --in-place overlay.qcow2 virt-sparsify: error: disk image 'overlay.qcow2' appears to have a backing file. You should use copying mode sparsification for this file (see the virt-sparsify(1) manual). Thanks: Yaniv Kaul --- sparsify/in_place.ml | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/sparsify/in_place.ml b/sparsify/in_place.ml index 3a2b32c..669c639 100644 --- a/sparsify/in_place.ml +++ b/sparsify/in_place.ml @@ -30,6 +30,11 @@ open Cmdline module G = Guestfs let rec run disk format ignores machine_readable zeroes = + (* If the disk has a backing file, refuse to run (RHBZ#1277705). *) + if (new G.guestfs ())#disk_has_backing_file disk then + error (f_"disk image '%s' appears to have a backing file. You should use copying mode sparsification for this file (see the virt-sparsify(1) manual).") + disk; + (* Connect to libguestfs. *) let g = new G.guestfs () in if trace () then g#set_trace true;