On Tue, Feb 10, 2015 at 02:54:39PM +0100, Kevin Wolf wrote: > 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 <kw...@redhat.com> --- > > > > > > > > > >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? >
Yes, exactly. I don't know if that will catch all the edge use case combinations (an image may have been created by Virtual PC, but formatted and used as a guest under Hyper-V, etc..). But it seems like a reasonable approach that improves the current situation.