Re: [Qemu-devel] [Qemu-block] Request for clarification on qemu-img convert behavior zeroing target host_device

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Dec 14, 2018 at 12:52:34PM +0000, De Backer, Fred (Nokia - BE/Antwerp) wrote:
> >Of course, we should also think about the other problem you mentioned, related to copying a smaller image to a larger block device. Does this require zeroing the parts after the image or should we leave them alone?
> >
> >I'd tend to say that since you're passing the whole block device as a target to 'qemu-img convert', and the whole block device will be visible to a guest run with the same block device configuration, we should indeed zero out the whole device. But then we would declare the F27 behaviour buggy and this case would stay slow (it would become slightly faster because we avoid the double writes, but we wouldn't save the writes to the unused space).
> 
> As long as it's outside the region of the source image I think you can leave it alone. Similar to deleting a file on a disk also doesn't zero out the sectors that were used to store that file before.

It's really nothing at all like that case.  Kevin is right the only
sensible thing to do is to zero-extend the image to the full size of
the target (in the absence of the user instructing qemu-img to do
something else).

Rich.

> >Or we could just refuse to convert if source and target aren't the same size. Then you would have to use a raw filter driver to select a part from the target image with the offset/size options. But this would break backwards compatibility, and its use is not very intuitive either.
> 
> Going that path would certainly break the way Openstack Ironic project uses qemu-img via the ironic-python-agent to image baremetal servers. And I can imagine there are other cases out there using qemu-img to write out disk images to blockdevices.
> 
> Fred

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