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

[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.

Kevin