Libguestfs - Jun 2013 - Re: How to use libguestfs access LVM as non-root user?

On Fri, May 31, 2013 at 7:25 PM, Richard W.M. Jones
<rjones@redhat.com>wrote:
> On Fri, May 31, 2013 at 06:39:53PM +0800, Qiu Yu wrote:
> > [00098ms] /usr/libexec/qemu-kvm \
> >     -global virtio-blk-pci.scsi=off \
> >     -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio \
> >     -nodefconfig \
> >     -enable-kvm \
> >     -nodefaults \
> >     -nographic \
> >     -m 500 \
> >     -no-reboot \
> >     -device virtio-serial \
> >     -serial stdio \
> >     -chardev
socket,path=/tmp/libguestfssaaw6T/guestfsd.sock,id=channel0
> \
> >     -device
> virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
> >     -kernel /var/tmp/.guestfs-501/kernel.30285 \
> >     -initrd /var/tmp/.guestfs-501/initrd.30285 \
> >     -append 'panic=1 console=ttyS0 udevtimeout=300 no_timer_check
> acpi=off
> > printk.time=1 cgroup_disable=memory selinux=0 guestfs_verbose=1
> > TERM=screen-bce ' \
> >     -drive
> >
>
file=/var/tmp/.guestfs-501/root.30285,snapshot=on,if=virtio,cache=unsafeqemu-kvm:
> > -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio: could not
> open
> > disk image /dev/xenvg/123: Invalid argument
>
> I'm assuming it's because the format is wrong (ie. not qcow2 but
raw).
>
> The error message is a little bit obscure and could be better, but we
> do rely on qemu printing something sensible instead of just "Invalid
> argument".
>
>
Oh, yes. It's indeed the wrong 'format' argument causing the issue.
Sorry
I'm not familiar with libguestfs, 'invalid argument' should be an
obvious
hint for me to follow on.

For others who might interests, if you use qcow2 image with OpenStack and
choose LVM as image backend, nova libvirt driver will convert it raw when
creating a new instance. However, data injection using libguestfs will
still assuming the it is qcow2 format, hence hit the above issue. Not sure
it is a bug or not, probably one should specify "use_cow_images =
False" to
avoid the problem in this case.

--
Qiu Yu

On Sat, Jun 01, 2013 at 01:28:20PM +0800, Qiu Yu wrote:
> For others who might interests, if you use qcow2 image with OpenStack and
> choose LVM as image backend, nova libvirt driver will convert it raw when
> creating a new instance. However, data injection using libguestfs will
> still assuming the it is qcow2 format, hence hit the above issue. Not sure
> it is a bug or not, probably one should specify "use_cow_images =
False" to
> avoid the problem in this case.
Yes, this sounds like a Nova bug.

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

On 06/01/2013 02:27 PM, Richard W.M. Jones wrote:
> On Sat, Jun 01, 2013 at 01:28:20PM +0800, Qiu Yu wrote:
>> For others who might interests, if you use qcow2 image with OpenStack
and
>> choose LVM as image backend, nova libvirt driver will convert it raw
when
>> creating a new instance.
IIUC, that's the reason, why it converts to raw --
https://github.com/openstack/nova/commit/ff9d353b

-------------------------
"This uses 'qemu-img' to convert images that are not 'raw'
to be 'raw'.
By doing so, it
 a.) refuses to run uploaded images that have a backing image reference
     (LP: #853330, CVE-2011-3147)
 b.) ensures that when FLAGS.use_cow_images is False, and the libvirt
     xml written specifies 'driver_type="raw"' that the disk
referenced
     is also raw format. (LP: #837102)
 c.) removes compression that might be present to avoid cpu bottlenecks
     (LP: #837100)

It does have the negative side affect of using more space in the case where
the user uploaded a qcow2 (or other advanced image format) that could have
been used directly by the hypervisor.  That could, later, be remedied by
another 'qemu-img convert' being done to the "preferred"
format of the
hypervisor."
-------------------------

It also does it during nova-image-create. A small write-up on how/why --
http://kashyapc.wordpress.com/2013/03/11/openstack-nova-image-create-under-the-hood/


 However, data injection using libguestfs will
>> still assuming the it is qcow2 format, hence hit the above issue. Not
sure
>> it is a bug or not, probably one should specify "use_cow_images =
False" to
>> avoid the problem in this case.
> 
> Yes, this sounds like a Nova bug.
> 
> Rich.
> 

-- 
/kashyap