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[Qemu-devel] [Bug 1368815] Re: qemu-img convert intermittently corrupts
From: |
Michael Steffens |
Subject: |
[Qemu-devel] [Bug 1368815] Re: qemu-img convert intermittently corrupts output images |
Date: |
Mon, 15 Sep 2014 10:08:41 -0000 |
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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