From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37754) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLpzq-0004xJ-SJ for qemu-devel@nongnu.org; Thu, 12 Feb 2015 04:23:44 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YLpzl-0003PS-QQ for qemu-devel@nongnu.org; Thu, 12 Feb 2015 04:23:42 -0500 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:59751 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YLpzl-0003P5-Fx for qemu-devel@nongnu.org; Thu, 12 Feb 2015 04:23:37 -0500 Message-ID: <54DC7112.30809@kamp.de> Date: Thu, 12 Feb 2015 10:23:30 +0100 From: Peter Lieven MIME-Version: 1.0 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> In-Reply-To: <20150210145329.GG5202@noname.str.redhat.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit 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: Kevin Wolf Cc: Jeff Cody , qemu-devel@nongnu.org, stefanha@redhat.com 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. Peter