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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: Thu, 12 Feb 2015 11:06:38 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 12.02.2015 um 11:02 hat Peter Lieven geschrieben:
> Am 12.02.2015 um 10:58 schrieb Kevin Wolf:
> >Am 12.02.2015 um 10:23 hat Peter Lieven geschrieben:
> >>Am 10.02.2015 um 15:53 schrieb Kevin Wolf:
> >>>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? ;-)
> >>I knew that this would happen ;-)
> >>
> >>Regarding the C/H/S calculation. I was just wondering if we should
> >>not set this to maximum (=invalid?) for all newly created images.
> >>That is what disk2vhd does.
> >CHS is what Virtual PC relies on. So I guess if you did that, you
> >would render images unusable by it. Are you sure that disk2vhd does this
> >always? I would have thought that it only does it for large images.
> At least 2.0.1 (latest available version) does this as well as the version
> that I used when I added the hack for d2v creator.
> Virtual PC would not be able to use images we create with qemu-img create
> if we use footer->size (which I suppose to reanme to footer->cur_size, btw)
> to calculate bs->total_sectors because we might write data to the end of
> the image which gets truncated in CHS format.

These kinds of problems are why I'd like to keep CHS and size always
consistent when creating an image with qemu-img.


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