Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description

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

On 02/26/2015 04:44 PM, Fam Zheng wrote:
> On Thu, 02/26 14:38, Wen Congyang wrote:
>> On 02/25/2015 10:46 AM, Fam Zheng wrote:
>>> On Tue, 02/24 15:50, Wen Congyang wrote:
>>>> On 02/12/2015 04:44 PM, Fam Zheng wrote:
>>>>> On Thu, 02/12 15:40, Wen Congyang wrote:
>>>>>> On 02/12/2015 03:21 PM, Fam Zheng wrote:
>>>>>>> Hi Congyang,
>>>>>>>
>>>>>>> On Thu, 02/12 11:07, Wen Congyang wrote:
>>>>>>>> +== Workflow ==
>>>>>>>> +The following is the image of block replication workflow:
>>>>>>>> +
>>>>>>>> +        +----------------------+            +------------------------+
>>>>>>>> +        |Primary Write Requests|            |Secondary Write Requests|
>>>>>>>> +        +----------------------+            +------------------------+
>>>>>>>> +                  |                                       |
>>>>>>>> +                  |                                      (4)
>>>>>>>> +                  |                                       V
>>>>>>>> +                  |                              /-------------\
>>>>>>>> +                  |      Copy and Forward        |             |
>>>>>>>> +                  |---------(1)----------+       | Disk Buffer |
>>>>>>>> +                  |                      |       |             |
>>>>>>>> +                  |                     (3)      \-------------/
>>>>>>>> +                  |                 speculative      ^
>>>>>>>> +                  |                write through    (2)
>>>>>>>> +                  |                      |           |
>>>>>>>> +                  V                      V           |
>>>>>>>> +           +--------------+           +----------------+
>>>>>>>> +           | Primary Disk |           | Secondary Disk |
>>>>>>>> +           +--------------+           +----------------+
>>>>>>>> +
>>>>>>>> +    1) Primary write requests will be copied and forwarded to Secondary
>>>>>>>> +       QEMU.
>>>>>>>> +    2) Before Primary write requests are written to Secondary disk, the
>>>>>>>> +       original sector content will be read from Secondary disk and
>>>>>>>> +       buffered in the Disk buffer, but it will not overwrite the existing
>>>>>>>> +       sector content in the Disk buffer.
>>>>>>>
>>>>>>> I'm a little confused by the tenses ("will be" versus "are") and terms. I am
>>>>>>> reading them as "s/will be/are/g"
>>>>>>>
>>>>>>> Why do you need this buffer?
>>>>>>
>>>>>> We only sync the disk till next checkpoint. Before next checkpoint, secondary
>>>>>> vm write to the buffer.
>>>>>>
>>>>>>>
>>>>>>> If both primary and secondary write to the same sector, what is saved in the
>>>>>>> buffer?
>>>>>>
>>>>>> The primary content will be written to the secondary disk, and the secondary content
>>>>>> is saved in the buffer.
>>>>>
>>>>> I wonder if alternatively this is possible with an imaginary "writable backing
>>>>> image" feature, as described below.
>>>>>
>>>>> When we have a normal backing chain,
>>>>>
>>>>>                {virtio-blk dev 'foo'}
>>>>>                          |
>>>>>                          |
>>>>>                          |
>>>>>     [base] <- [mid] <- (foo)
>>>>>
>>>>> Where [base] and [mid] are read only, (foo) is writable. When we add an overlay
>>>>> to an existing image on top,
>>>>>
>>>>>                {virtio-blk dev 'foo'}        {virtio-blk dev 'bar'}
>>>>>                          |                              |
>>>>>                          |                              |
>>>>>                          |                              |
>>>>>     [base] <- [mid] <- (foo)  <---------------------- (bar)
>>>>>
>>>>> It's important to make sure that writes to 'foo' doesn't break data for 'bar'.
>>>>> We can utilize an automatic hidden drive-backup target:
>>>>>
>>>>>                {virtio-blk dev 'foo'}                                    {virtio-blk dev 'bar'}
>>>>>                          |                                                          |
>>>>>                          |                                                          |
>>>>>                          v                                                          v
>>>>>
>>>>>     [base] <- [mid] <- (foo)  <----------------- (hidden target) <--------------- (bar)
>>>>>
>>>>>                          v                              ^
>>>>>                          v                              ^
>>>>>                          v                              ^
>>>>>                          v                              ^
>>>>>                          >>>> drive-backup sync=none >>>>
>>>>>
>>>>> So when guest writes to 'foo', the old data is moved to (hidden target), which
>>>>> remains unchanged from (bar)'s PoV.
>>>>>
>>>>> The drive in the middle is called hidden because QEMU creates it automatically,
>>>>> the naming is arbitrary.
>>>>
>>>> I don't understand this. In which function, the hidden target is created automatically?
>>>>
>>>
>>> It's to be determined. This part is only in my mind :)
>>
>> What about this:
>> -drive file=nbd-target,if=none,id=nbd-target0 \
>> -drive file=active-disk,if=virtio,driver=qcow2,backing.file.filename=hidden-disk,backing.driver=qcow2,backing.backing=nbd-target0
>>
> 
> It's close. I suppose backing.backing is referencing another drive as its
> backing_hd, then you cannot have the other backing.file.* option - they
> conflict. It would be something along:
> 
> -drive file=nbd-target,if=none,id=nbd-target0 \
> -drive file=hidden-disk,if=none,id=hidden0,backing.backing=nbd-target0 \
> -drive file=active-disk,if=virtio,driver=qcow2,backing.backing=hidden0
> 
> Or for simplicity, s/backing.backing=/backing=/g

If using backing=drive_id, backing.backing and backing.file.* are not conflict.
backing.backing=$drive_id means that: backing file's backing file's id is $drive_id.

> 
> Yes, adding these "backing=$drive_id" option is also exactly what we expect
> in order to support image-fleecing, but we haven't figured how to allow that
> without breaking other qmp operations like block jobs, etc.

I don't understand this. In which case, qmp operations will be broken? Can you give
me some examples?

Thanks
Wen Congyang

> 
> Fam
> .
>