From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40331) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLAxN-000597-FI for qemu-devel@nongnu.org; Tue, 10 Feb 2015 08:34:26 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YLAxK-0007Ah-8I for qemu-devel@nongnu.org; Tue, 10 Feb 2015 08:34:25 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58410) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLAxJ-0007AT-RX for qemu-devel@nongnu.org; Tue, 10 Feb 2015 08:34:22 -0500 Date: Tue, 10 Feb 2015 14:34:14 +0100 From: Kevin Wolf Message-ID: <20150210133414.GE5202@noname.str.redhat.com> References: <1423498163-2001-1-git-send-email-kwolf@redhat.com> <54D9EE7A.9050802@kamp.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54D9EE7A.9050802@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: jcody@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com 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. Kevin