From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56164) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XSSaB-0002js-1Q for qemu-devel@nongnu.org; Fri, 12 Sep 2014 11:16:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XSSa5-0003ak-OL for qemu-devel@nongnu.org; Fri, 12 Sep 2014 11:16:18 -0400 Received: from indium.canonical.com ([91.189.90.7]:35183) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XSSa5-0003ad-Jo for qemu-devel@nongnu.org; Fri, 12 Sep 2014 11:16:13 -0400 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.76 #1 (Debian)) id 1XSSa4-0004qw-Le for ; Fri, 12 Sep 2014 15:16:12 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 8A6F22E80BF for ; Fri, 12 Sep 2014 15:16:12 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Fri, 12 Sep 2014 15:08:02 -0000 From: Michael Steffens Sender: bounces@canonical.com References: <20140912145823.442.8812.malonedeb@gac.canonical.com> Message-Id: <20140912150803.32380.67323.launchpad@gac.canonical.com> Errors-To: bounces@canonical.com Subject: [Qemu-devel] [Bug 1368815] Re: qemu-img convert intermittently corrupts output images Reply-To: Bug 1368815 <1368815@bugs.launchpad.net> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org ** Also affects: nova Importance: Undecided Status: New -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1368815 Title: qemu-img convert intermittently corrupts output images Status in OpenStack Compute (Nova): Triaged Status in QEMU: New Bug description: -- Found in releases qemu-2.0.0, qemu-2.0.2, qemu-2.1.0. Tested on Ubuntu 14.04 using Ext4 filesystems. The command qemu-img convert -O raw inputimage.qcow2 outputimage.raw intermittently creates corrupted output images, when the input image is not yet fully synchronized to disk. While the issue has actually been discovered in operation of of OpenStack nova, it can be reproduced "easily" on command line using cat $SRC_PATH > $TMP_PATH && $QEMU_IMG_PATH convert -O raw $TMP_PATH $DST_PATH && cksum $DST_PATH on filesystems exposing this behavior. (The difficult part of this exercise is to prepare a filesystem to reliably trigger this race. On my test machine some filesystems are affected while other aren't, and unfortunately I haven't found the relevant difference between them, yet. Possible it's timing issues completely out of userspace control ...) The root cause, however, is the same as in http://lists.gnu.org/archive/html/coreutils/2011-04/msg00069.html and it can be solved the same way as suggested in http://lists.gnu.org/archive/html/coreutils/2011-04/msg00102.html In qemu, file block/raw-posix.c use the FIEMAP_FLAG_SYNC, i.e change f.fm.fm_flags =3D 0; to f.fm.fm_flags =3D FIEMAP_FLAG_SYNC; As discussed in the thread mentioned above, retrieving a page cache coherent map of file extents is possible only after fsync on that file. See also https://bugs.launchpad.net/nova/+bug/1350766 In that bug report filed against nova, fsync had been suggested to be performed by the framework invoking qemu-img. However, as the choice of fiemap -- implying this otherwise unneeded fsync of a temporary file -- is not made by the caller but by qemu-img, I agree with the nova bug reviewer's objection to put it into nova. The fsync should instead be triggered by qemu-img utilizing the FIEMAP_FLAG_SYNC, specifically intended for that purpose. To manage notifications about this bug go to: https://bugs.launchpad.net/nova/+bug/1368815/+subscriptions