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

[Michael Steffens]

It seems the dust settles a bit: Found the relevant difference between
my various filesystems, and how to reproduce the failure: Susceptible
filesystems don't have the extent feature of ext4 enabled.

You can create such a filesystem using

  mke2fs -t ext4 -O ^extent /dev/...
  mount /mnt /dev/...
 
Adapting the command line example provided above you can see

  rm -f /mnt/tmp.qcow2
  cat $SRC_PATH > /mnt/tmp.qcow2 && qemu-img convert -O raw  /mnt/tmp.qcow 
/mnt/tmp.qcow
  cksum  /mnt/tmp.qcow

creating corrupt (usually nullified) result images. By inserting a sleep
of at least 33 seconds between the cat command and the qemu-img
invocation I'm getting proper output.

To me it's unclear now, where the actual defect is located. Creating
ext4 filesystems with certain features disabled (such as the exetent
tree) is apparently supported and ok. Is the fiemap ioctl supposed to
handle this gracefully, for example by assuming FIEMAP_FLAG_SYNC in
absence of an extent tree? Or are clients such as qemu-img supposed to
always FIEMAP_FLAG_SYNC to be safe?

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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 = 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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