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Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img conv
From: |
Richard W.M. Jones |
Subject: |
Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device |
Date: |
Fri, 14 Dec 2018 13:10:11 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Eric Blake, 2018/12/13
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Kevin Wolf, 2018/12/13
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Eric Blake, 2018/12/13
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, De Backer, Fred (Nokia - BE/Antwerp), 2018/12/13
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Nir Soffer, 2018/12/13
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, De Backer, Fred (Nokia - BE/Antwerp), 2018/12/14
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, De Backer, Fred (Nokia - BE/Antwerp), 2018/12/14
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Kevin Wolf, 2018/12/14
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, De Backer, Fred (Nokia - BE/Antwerp), 2018/12/14
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device,
Richard W.M. Jones <=
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Daniel P . Berrangé, 2018/12/14