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Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img conv
From: |
De Backer, Fred (Nokia - BE/Antwerp) |
Subject: |
Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device |
Date: |
Fri, 14 Dec 2018 12:52:34 +0000 |
>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.
>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
- 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) <=
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Richard W.M. Jones, 2018/12/14
- Re: [Qemu-block] [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device, Daniel P . Berrangé, 2018/12/14