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

[Stefan Hajnoczi]

On Fri, Mar 27, 2015 at 9:14 AM, Wen Congyang <address@hidden> 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 <address@hidden> wrote:
>>>> On 03/25/2015 05:50 PM, Juan Quintela wrote:
>>>>> zhanghailiang <address@hidden> 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.

Stefan