[Qemu-devel] [Bug 1368815] Re: qemu-img convert intermittently corrupts output images

[Serge Hallyn]

Thanks for the information.  Looks like we can apply these in debian
too.

** Also affects: qemu (Ubuntu Utopic)
   Importance: High
       Status: Triaged

** Also affects: qemu (Ubuntu Vivid)
   Importance: Undecided
       Status: New

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Title:
  qemu-img convert intermittently corrupts output images

Status in OpenStack Compute (Nova):
  In Progress
Status in QEMU:
  In Progress
Status in “qemu” package in Ubuntu:
  Triaged
Status in “qemu” source package in Trusty:
  Triaged
Status in “qemu” source package in Utopic:
  Triaged
Status in “qemu” source package in Vivid:
  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 = 0;

  to

      f.fm.fm_flags = 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.

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