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

[Eric Blake]

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.

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 at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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