From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43963) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLqfa-0001xY-It for qemu-devel@nongnu.org; Thu, 12 Feb 2015 05:06:51 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YLqfX-0000kH-Cw for qemu-devel@nongnu.org; Thu, 12 Feb 2015 05:06:50 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42705) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLqfX-0000kA-5Z for qemu-devel@nongnu.org; Thu, 12 Feb 2015 05:06:47 -0500 Date: Thu, 12 Feb 2015 11:06:38 +0100 From: Kevin Wolf Message-ID: <20150212100638.GF4189@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> <20150210145329.GG5202@noname.str.redhat.com> <54DC7112.30809@kamp.de> <20150212095821.GE4189@noname.str.redhat.com> <54DC7A1E.6070405@kamp.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54DC7A1E.6070405@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 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 --- > >>>>>>>>> > >>>>>>>>>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. Kevin