Re: [Qemu-devel] Several questions about " [PATCH v3 0/5] virtio-9p: hotplug and migration support"

[wangyan]

On 2019/3/11 17:33, Greg Kurz wrote:
> On Tue, 5 Mar 2019 19:28:37 +0800
> wangyan <address@hidden> wrote:
>
>> I am very happy for your reply. 
>>
>> I am very interested in your old WIP and want to take a look. 
>>
>> Our project still use 9pfs, and it needs to live migrate the
>> device while it is being used by the guest, is there any advice
>> for us to make it come true or any WIP about this? 
>>
> It isn't a trivial task, really. The first thing to address is to be able
> to drain outstanding I/O requests. Then, about migration itself, the current
> idea was to close files at pre-save so that they get re-open automatically
> on the target if migration succeeds or on the source if migration fails.
> Then we get to some more gory details: files open with O_EXCL and unlinked
> open files...
I think about the point "unlinked open files...", and have a method to solve 
this problem.
For example, if the guest opens a file, write some data in it, unlinks the file 
but keeps the fd,
then we can use the fd to read all content in the file and put it to the target 
host, and create
a file to save these content, and then unlink the file after the guest opens 
this file in the target
host. It can recover to the original state.

But this is not a good idea, it does a lot of meaningless work. Do you have any 
good idea for this?////
>> On 2019/2/26 17:11, Greg Kurz wrote:
>>> On Tue, 26 Feb 2019 11:17:16 +0800
>>> wangyan <address@hidden> wrote:
>>>  
>>>> Dear Kurz,
>>>>
>>>> Good day.
>>>>
>>>> This is Yan Wang from china. Glad to contact with you.
>>>>  
>>> Hi Yan Wang,
>>>  
>>>> I found your patch "[Qemu-devel] [PATCH v3 0/5] virtio-9p: hotplug and 
>>>> migration support" on the internet. Recently, our project need to use the 
>>>> feature of 9pfs living migration, and I want to know if you have the plan 
>>>> to merge the patch to the master branch of qemu, or do you have any other 
>>>> plans about the migration feature?
>>>>  
>>> AFAIK some of these patches have already been merged.
>>>
>>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=4652f1640e029e1f2433fa77ba6af285c7cd923a
>>> https://git.qemu.org/?p=qemu.git;a=commitdiff;h=6cecf093735f2e5af7d0e29d957350320044e354
>>>
>>> These are enough to live-migrate a quiescent virtio-9p device, ie,
>>> only if the 9p filesystem is NOT mounted in the guest.
>>>
>>> If your need is to live migrate the device while it is being used by
>>> the guest, then this is a different story. Basically, this requires
>>> to quiesce the device so that its state can be captured. Also, some
>>> state belongs to the host and maybe hard to capture/migrate. For
>>> example, if the guest opens a file, write some data in it, unlinks the
>>> file but keeps the fd, it expects to be able to read the same data
>>> back, even after migration to some other host system.
>>>
>>> 9pfs is no longer a priority for anyone in the industry. I have no
>>> plan to make this go forward, an not much time to work on it. But
>>> if you want to have a look, I guess I can share some old WIP I have
>>> somewhere.
>>>  
>>>> Sincerely,
>>>>
>>>> Yan Wang
>>>>  
>>> Cheers,
>>>
>>> --
>>> Greg
>>>
>>> .
>>>  
>>
>
> .
>