Re: [Qemu-devel] [Migration Bug? ] Occasionally, the content of VM's memory is inconsistent between Source and Destination of migration

[Wen Congyang <wency@cn.fujitsu.com>]

On 03/27/2015 05:57 PM, Stefan Hajnoczi wrote:
> On Fri, Mar 27, 2015 at 9:14 AM, Wen Congyang <wency@cn.fujitsu.com> wrote:
>> On 03/27/2015 04:56 PM, Stefan Hajnoczi wrote:
>>> On Thu, Mar 26, 2015 at 11:29:43AM +0100, Juan Quintela wrote:
>>>> Wen Congyang <wency@cn.fujitsu.com> wrote:
>>>>> On 03/25/2015 05:50 PM, Juan Quintela wrote:
>>>>>> zhanghailiang <zhang.zhanghailiang@huawei.com> wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> We found that, sometimes, the content of VM's memory is
>>>>>>> inconsistent between Source side and Destination side
>>>>>>> when we check it just after finishing migration but before VM continue to Run.
>>>>>>>
>>>>>>> We use a patch like bellow to find this issue, you can find it from affix,
>>>>>>> and Steps to reprduce:
>>>>>>>
>>>>>>> (1) Compile QEMU:
>>>>>>>  ./configure --target-list=x86_64-softmmu  --extra-ldflags="-lssl" && make
>>>>>>>
>>>>>>> (2) Command and output:
>>>>>>> SRC: # x86_64-softmmu/qemu-system-x86_64 -enable-kvm -cpu
>>>>>>> qemu64,-kvmclock -netdev tap,id=hn0-device
>>>>>>> virtio-net-pci,id=net-pci0,netdev=hn0 -boot c -drive
>>>>>>> file=/mnt/sdb/pure_IMG/sles/sles11_sp3.img,if=none,id=drive-virtio-disk0,cache=unsafe
>>>>>>> -device
>>>>>>> virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
>>>>>>> -vnc :7 -m 2048 -smp 2 -device piix3-usb-uhci -device usb-tablet
>>>>>>> -monitor stdio
>>>>>>
>>>>>> Could you try to reproduce:
>>>>>> - without vhost
>>>>>> - without virtio-net
>>>>>> - cache=unsafe is going to give you trouble, but trouble should only
>>>>>>   happen after migration of pages have finished.
>>>>>
>>>>> If I use ide disk, it doesn't happen.
>>>>> Even if I use virtio-net with vhost=on, it still doesn't happen. I guess
>>>>> it is because I migrate the guest when it is booting. The virtio net
>>>>> device is not used in this case.
>>>>
>>>> Kevin, Stefan, Michael, any great idea?
>>>
>>> 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?

Thanks
Wen Congyang

> 
> Stefan
> .
>