[fedora-virt] Problem when migrating qcow2 image from F20 to CentOS6

[Kevin Wolf]

Am 14.04.2014 um 12:33 hat Pavel Lisý geschrieben:
> Hello
> 
> I've made virtual guest on Fedora 20 (x86_64) within qcow2 image file
> but after migration to CentOS 6.5 I've got this error:
> 
> qemu-kvm: -drive file=/var/lib/libvirt/images/questfile.qcow2,if=none,id=drive-virtio-disk0,format=qcow2: 'drive-virtio-disk0' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3
> 
> similar error I've got for qemu-img:
> # qemu-img info /var/lib/libvirt/images/questfile.qcow2
> 'image' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3
> 
> On F20 it returns:
> # qemu-img info /var/lib/libvirt/images/questfile.qcow2
> image: /var/lib/libvirt/images/rhel6.qcow2
> file format: qcow2
> virtual size: 32G (34359738368 bytes)
> disk size: 4.7G
> cluster_size: 65536
> 
> 
> Have I missed something? 
> 
> Is it qcow2 on Fedora different from CentOS6? Is there any possibility
> (on Fedora 20) to convert it to backward compatible format?

qemu versions starting with 1.1 can use new qcow2 features which require
an incompatible on-disk format. Between version 1.1 and 1.6, they needed
to be specified explicitly during image creation, like this:

    qemu-img create -f qcow2 -o compat=1.1 test.qcow2 8G

Starting with qemu 1.7, compat=1.1 became the default, so that newly
created images can't be read by older qemu versions by default. If you
need to read them in older version, you now need to be explicit about
using the old format:

    qemu-img create -f qcow2 -o compat=0.10 test.qcow2 8G

With the same release, qemu 1.7, a new qemu-img subcommand was
introduced that allows converting between both versions, so you can
downgrade your existing v3 image to the format known by RHEL 6 like
this:

    qemu-img amend -f qcow2 -o compat=0.10 test.qcow2

However, it looked to me as if F20 still had qemu 1.6, so you shouldn't
be seeing any of this, and your qemu-img may not have the 'amend'
subcommand. Maybe one of the package maintainers can comment on any
patches that F20 has on top of 1.6?

Kevin