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
> .
>
