From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46169) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZC3qk-00061z-Rv for qemu-devel@nongnu.org; Mon, 06 Jul 2015 06:42:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZC3qh-0006Xq-7v for qemu-devel@nongnu.org; Mon, 06 Jul 2015 06:42:10 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:46902 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZC3qg-0006VW-Tq for qemu-devel@nongnu.org; Mon, 06 Jul 2015 06:42:07 -0400 Message-ID: <559A5B79.4010707@kamp.de> Date: Mon, 06 Jul 2015 12:42:01 +0200 From: Peter Lieven MIME-Version: 1.0 References: <559ABE850200006600045FF0@relay2.provo.novell.com> In-Reply-To: <559ABE850200006600045FF0@relay2.provo.novell.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] vpc size reporting problem List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Chun Yan Liu , qemu-devel@nongnu.org Cc: kwolf@redhat.com, mreitz@redhat.com Am 06.07.2015 um 11:44 schrieb Chun Yan Liu: > While testing with a 1GB VHD file created on win7, found that the VHD file > size reported on Windows is different from that is reported by qemu-img > info or within a Linux KVM guest. > > Created a dynamic VHD file on win7, on Windows, it is reported 1024MB > (2097152 sectors). But with qemu-img info or within a Linux KVM guest, > it is reported 1023MB (2096640 sectors). > > The values in the footer_buf are as follows: > creator_app: "win " > cylinders: 0x820 (2080) > heads: 0x10 (16) > cyl/sec: 0x3f (63) > current_size: 0x40000000 (1G) > > So, if using current_size, it's correct; but using CHS will get a smaller size. > > Should we add a check in this case and use "current_size" instead of > CHS? As far as I remember the issue was and still is that there is no official spec that says use current_size in case A and CHS in case B. If currrent_size is greater than CHS and Windows would use CHS (we don't know that) we might run into issues if Qemu uses current_size. In this cas we would write data beyond the end of the container (from Windows perspective). > > BTW, before commit 0444dceee, there is a similar check for 'd2v', > if creator_app is 'd2v', using "size" instead of CHS. But in commit > 0444dceee, this check is removed. To me, the new check and 'd2v' > check seem to be two different cases, why removing 'd2v' check? d2v always writes a magic combination of 65535x16x255 for CHS. So commit 0444dceee just changed the behaviour to always use current_size in case CHS is 65535x16x255 (including d2v). I personally wouldn't mind to always use current_size that is what e.g. VirtualBox does. Or use current_size if it is greater than the size derived from CHS. But this might break things. Peter