From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40243) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLCCD-0001ne-4e for qemu-devel@nongnu.org; Tue, 10 Feb 2015 09:53:50 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YLCC8-0004ho-FZ for qemu-devel@nongnu.org; Tue, 10 Feb 2015 09:53:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34810) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLCC8-0004hk-7v for qemu-devel@nongnu.org; Tue, 10 Feb 2015 09:53:44 -0500 Date: Tue, 10 Feb 2015 15:53:29 +0100 From: Kevin Wolf Message-ID: <20150210145329.GG5202@noname.str.redhat.com> References: <1423498163-2001-1-git-send-email-kwolf@redhat.com> <54D9EE7A.9050802@kamp.de> <20150210133414.GE5202@noname.str.redhat.com> <20150210134242.GB19775@localhost.localdomain> <20150210135439.GF5202@noname.str.redhat.com> <54DA0EEA.7050908@kamp.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54DA0EEA.7050908@kamp.de> Subject: Re: [Qemu-devel] [RFC PATCH] vpc: Ignore geometry for large images List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven Cc: Jeff Cody , qemu-devel@nongnu.org, stefanha@redhat.com 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 --- > >>>>> > >>>>>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? ;-) Kevin