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Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images
Date: Tue, 10 Feb 2015 15:53:29 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 10.02.2015 um 15:00 hat Peter Lieven geschrieben:
> Am 10.02.2015 um 14:54 schrieb Kevin Wolf:
> >Am 10.02.2015 um 14:42 hat Jeff Cody geschrieben:
> >>On Tue, Feb 10, 2015 at 02:34:14PM +0100, Kevin Wolf wrote:
> >>>Am 10.02.2015 um 12:41 hat Peter Lieven geschrieben:
> >>>>Am 09.02.2015 um 17:09 schrieb Kevin Wolf:
> >>>>>The CHS calculation as done per the VHD spec imposes a maximum
> >>>>>image size of ~127 GB. Real VHD images exist that are larger than
> >>>>>that.
> >>>>>
> >>>>>Apparently there are two separate non-standard ways to achieve
> >>>>>this: You could use more heads than the spec does - this is the
> >>>>>option that qemu-img create chooses.
> >>>>>
> >>>>>However, other images exist where the geometry is set to the
> >>>>>maximum (65536/16/255), but the actual image size is larger.
> >>>>>Until now, such images are truncated at 127 GB when opening them
> >>>>>with qemu.
> >>>>>
> >>>>>This patch changes the vpc driver to ignore geometry in this case
> >>>>>and only trust the size field in the header.
> >>>>>
> >>>>>Signed-off-by: Kevin Wolf <address@hidden> ---
> >>>>>
> >>>>>Peter, I'm replacing some of your code in the hope that the new
> >>>>>approach is more generally valid. Of course, I haven't tested if
> >>>>>your case with disk2vhd is still covered. Could you check this,
> >>>>>please?
> >>>>I checked this and found that disk2vhd always sets CHS to 65535ULL
> >>>>* 16 * 255 independed of the real size.
> >>>>
> >>>>But, as the conversion to CHS may have an error its maybe the best
> >>>>solution to ignore CHS completely and always derive total_sectors
> >>>>from footer->size unconditionally.
> >>>>
> >>>>I had a look at what virtualbox does and they only rely on
> >>>>footer->size. If they alter the size or create an image the write
> >>>>the new size into the footer and recalculate CHS by the formula
> >>>>found in the appendix of the original spec.
> >>>>
> >>>>Check vhdCreateImage, vhdOpen in
> >>>>http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Storage/VHD.cpp
> >>>>
> >>>>The original spec also says that CHS values purpose is the use in
> >>>>an ATA controller only.
> >>>The problem with just using footer->size back then when I
> >>>implemented this was that from the perspective of a VirtualPC guest
> >>>run in qemu, the size of its hard disk would change, which you don't
> >>>want either. Going from VPC to qemu would be ugly, but mostly
> >>>harmless as the disk only grows. But if you use an image in qemu
> >>>where the disk looks larger and then go back to VPC which respects
> >>>geometry, your data may be truncated.
> >>I believe the vpc "creator" field is different if the image was
> >>created by Virtual PC, versus created by Hyper-V ("vpc" and "win",
> >>respectively, I think).  Perhaps we could use that to infer a guest
> >>image came from VirtualPC, and thus not use footer->size in that
> >>scenario?
> >Right, I think we discussed that before. Do you remember the outcome of
> >that discussion? I seem to remember that we had a conclusion, but
> >apparently it was never actually implemented.
> >
> >Would your proposal be to special-case "vpc" to apply the geometry, and
> >everything else (including "win", "d2v" and "qemu") would use the footer
> >field?
> That sounds reasonable. In any case we have to fix qemu-img create
> to do not create out of spec geometry for images larger than 127G.
> It should set the correct footer->size and then calculate the geometry.

Do I understand correctly that you just volunteered to fix up that whole
thing? ;-)


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