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Re: [libvirt] bug: try to take disk snapshot for LVM2 Volume

On Mon, Nov 14, 2011 at 04:02:21PM -0700, Eric Blake wrote:
> On 11/13/2011 09:08 PM, MATSUDA, Daiki wrote:
> >> NACK.  There is nothing inherently wrong with the source file not being
> >> a qcow2 file.  The whole point of creating a runtime snapshot is that
> >> the original file (of _any_ format) becomes the backing file of a new
> >> qcow2 file, so that the original file now serves as the snapshot.
> >> Forbidding a live snapshot of a raw source file interferes with this intent.
> >>
> > 
> > I have some tested since you replied. Certainly other image file, e.g.
> > raw type, has no problem after snapshot is taken in qcow2 format.
> > 
> > But in the case that direct disk block, e.g. /dev/sdc, is given for the
> > guest OS, the same error occurs. It may not be accepted by qemu-kvm.
> I'm not following you.  As I already said, there's nothing wrong with a
> direct disk block as the backing file of a qcow2 image.
> > 
> > I do not understand which qemu-kvm or libvirt should be fixed. But at
> > least libvirt should be stop to take snapshot for block device with
> > following patch.
> I still say NACK.  Your patch is merely attempting to forbid a useful
> case, rather than fix a demonstrated problem.


I must admit that I don't understand the problem that you are
attempting to solve either.  I have reread the thread from the
beginning, but I don't understand what your goal is.  Could you
explain to us more about your use case?

Dave Allan

> If I recall correctly, you started this thread when you used 'virsh
> snapshot-create --disk-only' without arguments, and thus fell victim to
> virsh picking the snapshot name for you, but trying to pick that name
> under /dev instead of a more typical location for a non-device file.
> But that's a usage error in you not taking time to provide virsh with
> the alternate name to use for the qcow2 file, and not a flaw that needs
> correcting in libvirt itself.  Or even if there is a flaw in libvirt,
> the fix is not to forbid snapshots of a raw block device, but to be
> smarter about ensuring that any generated qcow2 file name is likely to
> be correct (a qcow2 file created under /dev probably only makes sense if
> the qcow2 data is being written atop a pre-existing block device, rather
> than trying to use open(O_CREAT) to create a regular file in /dev).
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org

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