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

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

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