Fix proposed to branch: master
Review: https://review.openstack.org/141259
** Changed in: cinder
Assignee: Tony Breeds (o-tony) => John Griffith (john-griffith)
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https://bugs.launchpad.net/bugs/1368815
Title:
qemu-img convert intermittently corrupts output images
Status in Cinder:
In Progress
Status in OpenStack Compute (Nova):
In Progress
Status in QEMU:
In Progress
Status in qemu package in Ubuntu:
Fix Released
Status in qemu source package in Trusty:
Fix Released
Status in qemu source package in Utopic:
Fix Released
Status in qemu source package in Vivid:
Fix Released
Bug description:
==========================================================
Impact: occasional image corruption (any format on local filesystem)
Test case: see the qemu-img command below
Regression potential: this cherrypicks a patch from upstream to a
not-insignificantly older qemu source tree. While the cherrypick seems sane,
it's possible that there are subtle interactions with the other delta. I'd
really like for a full qa-regression-test qemu testcase to be run against this
package.
==========================================================
-- 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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