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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <55152B4E.4050705@cn.fujitsu.com> (Wen Congyang's message of
	"Fri, 27 Mar 2015 18:05:02 +0800")
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Date: Fri, 27 Mar 2015 11:36:35 +0100
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Subject: Re: [Qemu-devel] [Migration Bug? ] Occasionally,
	the content of VM's memory is inconsistent between Source and
	Destination of migration
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To: Wen Congyang <wency@cn.fujitsu.com>
Cc: Kevin Wolf <kwolf@redhat.com>, hangaohuai@huawei.com, zhanghailiang <zhang.zhanghailiang@huawei.com>, Li Zhijian <lizhijian@cn.fujitsu.com>, Michael Tsirkin <mst@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, Stefan Hajnoczi <stefanha@redhat.com>, Amit Shah <amit.shah@redhat.com>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, David Gibson <david@gibson.dropbear.id.au>

Wen Congyang <wency@cn.fujitsu.com> wrote:
> On 03/27/2015 05:57 PM, Stefan Hajnoczi wrote:
>>>> You must use -drive cache=none if you want to use live migration.  It
>>>> should not directly affect memory during migration though.
>>>
>>> Otherwise, what will happen? If the user doesn't use cache=none, and
>>> tries to use live migration, qemu doesn't output any message or trigger
>>> an event to notify the user.
>> 
>> There is a risk that the destination host sees an inconsistent view of
>> the image file because the source was still accessing it towards the
>> end of migration.
>
> Does the flag BDRV_O_NOFLUSH cause it?

The biggest isue is reads.

Target: reads 1st block of disk
        it enters page cache of target
Source: reads and writes lots of blocks

Migration happens.

Target: rereads 1st block of disk, and gets stalled contents.

No amount of fdatasync(), syncs() whatever on source will fix this problem.


Later, Juan.