* [Qemu-devel] [PATCH] block: Flush image after open
@ 2011-03-09 16:15 Kevin Wolf
2011-03-09 17:27 ` Christoph Hellwig
0 siblings, 1 reply; 6+ messages in thread
From: Kevin Wolf @ 2011-03-09 16:15 UTC (permalink / raw)
To: qemu-devel; +Cc: kwolf
Quoting the bug report:
qemu ensures that guest writes and qemu metadata writes hit the disk
when necessary to prevent data corruption. However, if an image was
in host pagecache prior to starting qemu, for example after running
qemu-img convert, then nothing prevents writes from reaching the
disk out of order, potentially causing corruption.
I'm not entirely sure if there is a realistic case where we would get
corruption, but it's probably a case of better safe than sorry.
Reported-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
block.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/block.c b/block.c
index 8dea0b5..bf6892f 100644
--- a/block.c
+++ b/block.c
@@ -648,6 +648,14 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
bs->change_cb(bs->change_opaque, CHANGE_MEDIA);
}
+ /* Make sure that the image is consistent on disk */
+ if (!bdrv_is_read_only(bs)) {
+ ret = bdrv_flush(bs);
+ if (ret < 0) {
+ goto unlink_and_fail;
+ }
+ }
+
return 0;
unlink_and_fail:
--
1.7.2.3
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [Qemu-devel] [PATCH] block: Flush image after open
2011-03-09 16:15 [Qemu-devel] [PATCH] block: Flush image after open Kevin Wolf
@ 2011-03-09 17:27 ` Christoph Hellwig
2011-03-09 17:38 ` Anthony Liguori
0 siblings, 1 reply; 6+ messages in thread
From: Christoph Hellwig @ 2011-03-09 17:27 UTC (permalink / raw)
To: Kevin Wolf; +Cc: qemu-devel
On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
> Quoting the bug report:
>
> qemu ensures that guest writes and qemu metadata writes hit the disk
> when necessary to prevent data corruption. However, if an image was
> in host pagecache prior to starting qemu, for example after running
> qemu-img convert, then nothing prevents writes from reaching the
> disk out of order, potentially causing corruption.
>
> I'm not entirely sure if there is a realistic case where we would get
> corruption, but it's probably a case of better safe than sorry.
Except for SCSI with ordered tags (which we don't support) there are not
ordering guarantees in the storage protocols, and as such the above explanation
doesn't make any sense at all.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] block: Flush image after open
2011-03-09 17:27 ` Christoph Hellwig
@ 2011-03-09 17:38 ` Anthony Liguori
2011-03-21 12:23 ` Avi Kivity
0 siblings, 1 reply; 6+ messages in thread
From: Anthony Liguori @ 2011-03-09 17:38 UTC (permalink / raw)
To: Christoph Hellwig; +Cc: Kevin Wolf, qemu-devel, Avi Kivity
On 03/09/2011 11:27 AM, Christoph Hellwig wrote:
> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
>> Quoting the bug report:
>>
>> qemu ensures that guest writes and qemu metadata writes hit the disk
>> when necessary to prevent data corruption. However, if an image was
>> in host pagecache prior to starting qemu, for example after running
>> qemu-img convert, then nothing prevents writes from reaching the
>> disk out of order, potentially causing corruption.
>>
>> I'm not entirely sure if there is a realistic case where we would get
>> corruption, but it's probably a case of better safe than sorry.
> Except for SCSI with ordered tags (which we don't support) there are not
> ordering guarantees in the storage protocols, and as such the above explanation
> doesn't make any sense at all.
Even if there was, a guest shouldn't be relying on the ordering of a
write that comes from a non-guest.
I don't understand the failure scenario here.
Regards,
Anthony Liguori
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] block: Flush image after open
2011-03-09 17:38 ` Anthony Liguori
@ 2011-03-21 12:23 ` Avi Kivity
2011-03-21 13:02 ` Kevin Wolf
0 siblings, 1 reply; 6+ messages in thread
From: Avi Kivity @ 2011-03-21 12:23 UTC (permalink / raw)
To: Anthony Liguori; +Cc: Kevin Wolf, Christoph Hellwig, qemu-devel
On 03/09/2011 07:38 PM, Anthony Liguori wrote:
> On 03/09/2011 11:27 AM, Christoph Hellwig wrote:
>> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
>>> Quoting the bug report:
>>>
>>> qemu ensures that guest writes and qemu metadata writes hit the
>>> disk
>>> when necessary to prevent data corruption. However, if an image
>>> was
>>> in host pagecache prior to starting qemu, for example after
>>> running
>>> qemu-img convert, then nothing prevents writes from reaching the
>>> disk out of order, potentially causing corruption.
>>>
>>> I'm not entirely sure if there is a realistic case where we would get
>>> corruption, but it's probably a case of better safe than sorry.
>> Except for SCSI with ordered tags (which we don't support) there are not
>> ordering guarantees in the storage protocols, and as such the above
>> explanation
>> doesn't make any sense at all.
>
> Even if there was, a guest shouldn't be relying on the ordering of a
> write that comes from a non-guest.
>
> I don't understand the failure scenario here.
$ cp x.img y.img
$ qemu -drive file=y.img,cache=writeback
<read something from disk, send it over the network>
<no guest flushes>
<host crash>
The guest may expect that any or none of its writes hit the disk, but
that anything that it read from the disk, stays there.
--
error compiling committee.c: too many arguments to function
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] block: Flush image after open
2011-03-21 12:23 ` Avi Kivity
@ 2011-03-21 13:02 ` Kevin Wolf
2011-03-21 13:21 ` Avi Kivity
0 siblings, 1 reply; 6+ messages in thread
From: Kevin Wolf @ 2011-03-21 13:02 UTC (permalink / raw)
To: Avi Kivity; +Cc: Christoph Hellwig, qemu-devel
Am 21.03.2011 13:23, schrieb Avi Kivity:
> On 03/09/2011 07:38 PM, Anthony Liguori wrote:
>> On 03/09/2011 11:27 AM, Christoph Hellwig wrote:
>>> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
>>>> Quoting the bug report:
>>>>
>>>> qemu ensures that guest writes and qemu metadata writes hit the
>>>> disk
>>>> when necessary to prevent data corruption. However, if an image
>>>> was
>>>> in host pagecache prior to starting qemu, for example after
>>>> running
>>>> qemu-img convert, then nothing prevents writes from reaching the
>>>> disk out of order, potentially causing corruption.
>>>>
>>>> I'm not entirely sure if there is a realistic case where we would get
>>>> corruption, but it's probably a case of better safe than sorry.
>>> Except for SCSI with ordered tags (which we don't support) there are not
>>> ordering guarantees in the storage protocols, and as such the above
>>> explanation
>>> doesn't make any sense at all.
>>
>> Even if there was, a guest shouldn't be relying on the ordering of a
>> write that comes from a non-guest.
>>
>> I don't understand the failure scenario here.
>
> $ cp x.img y.img
> $ qemu -drive file=y.img,cache=writeback
> <read something from disk, send it over the network>
> <no guest flushes>
> <host crash>
>
> The guest may expect that any or none of its writes hit the disk, but
> that anything that it read from the disk, stays there.
Is it true for real hardware? Consider a reboot, you could still have
some data in a volatile disk write cache if the OS that ran before the
reboot hasn't flushed it.
Kevin
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Qemu-devel] [PATCH] block: Flush image after open
2011-03-21 13:02 ` Kevin Wolf
@ 2011-03-21 13:21 ` Avi Kivity
0 siblings, 0 replies; 6+ messages in thread
From: Avi Kivity @ 2011-03-21 13:21 UTC (permalink / raw)
To: Kevin Wolf; +Cc: Christoph Hellwig, qemu-devel
On 03/21/2011 03:02 PM, Kevin Wolf wrote:
> Am 21.03.2011 13:23, schrieb Avi Kivity:
> > On 03/09/2011 07:38 PM, Anthony Liguori wrote:
> >> On 03/09/2011 11:27 AM, Christoph Hellwig wrote:
> >>> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote:
> >>>> Quoting the bug report:
> >>>>
> >>>> qemu ensures that guest writes and qemu metadata writes hit the
> >>>> disk
> >>>> when necessary to prevent data corruption. However, if an image
> >>>> was
> >>>> in host pagecache prior to starting qemu, for example after
> >>>> running
> >>>> qemu-img convert, then nothing prevents writes from reaching the
> >>>> disk out of order, potentially causing corruption.
> >>>>
> >>>> I'm not entirely sure if there is a realistic case where we would get
> >>>> corruption, but it's probably a case of better safe than sorry.
> >>> Except for SCSI with ordered tags (which we don't support) there are not
> >>> ordering guarantees in the storage protocols, and as such the above
> >>> explanation
> >>> doesn't make any sense at all.
> >>
> >> Even if there was, a guest shouldn't be relying on the ordering of a
> >> write that comes from a non-guest.
> >>
> >> I don't understand the failure scenario here.
> >
> > $ cp x.img y.img
> > $ qemu -drive file=y.img,cache=writeback
> > <read something from disk, send it over the network>
> > <no guest flushes>
> > <host crash>
> >
> > The guest may expect that any or none of its writes hit the disk, but
> > that anything that it read from the disk, stays there.
>
> Is it true for real hardware? Consider a reboot, you could still have
> some data in a volatile disk write cache if the OS that ran before the
> reboot hasn't flushed it.
That's if RESET doesn't flush the cache. It's probably false for fc or
iscsi, but possibly true for IDE.
But it can't happen for a single-boot host, or a dual boot host with no
shared partitions.
--
error compiling committee.c: too many arguments to function
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2011-03-21 13:22 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
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2011-03-09 16:15 [Qemu-devel] [PATCH] block: Flush image after open Kevin Wolf
2011-03-09 17:27 ` Christoph Hellwig
2011-03-09 17:38 ` Anthony Liguori
2011-03-21 12:23 ` Avi Kivity
2011-03-21 13:02 ` Kevin Wolf
2011-03-21 13:21 ` Avi Kivity
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