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


From: Wen Congyang
Subject: Re: [Qemu-devel] [RFC PATCH 01/14] docs: block replication's description
Date: Wed, 25 Feb 2015 16:36:49 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

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 :)

Does hidden target is only used for COLO?

Thanks
Wen Congyang

> 
> Fam
> 
>>
>>>
>>> It is interesting because it is a more generalized case of image fleecing,
>>> where the (hidden target) is exposed via NBD server for data scanning (read
>>> only) purpose.
>>>
>>> More interestingly, with above facility, it is also possible to create a 
>>> guest
>>> visible live snapshot (disk 'bar') of an existing device (disk 'foo') very
>>> cheaply. Or call it shadow copy if you will.
>>>
>>> Back to the COLO case, the configuration will be very similar:
>>>
>>>
>>>                       {primary wr}                                          
>>>       {secondary vm}
>>>                             |                                               
>>>             |
>>>                             |                                               
>>>             |
>>>                             |                                               
>>>             |
>>>                             v                                               
>>>             v
>>>
>>>    [what] <- [ever] <- (nbd target) <------------ (hidden buf disk) 
>>> <------------- (active disk)
>>>
>>>                             v                              ^
>>>                             v                              ^
>>>                             v                              ^
>>>                             v                              ^
>>>                             >>>> drive-backup sync=none >>>>
>>>
>>> The workflow analogue is:
>>>
>>>>>> +    1) Primary write requests will be copied and forwarded to Secondary
>>>>>> +       QEMU.
>>>
>>> Primary write requests are forwarded to secondary QEMU as well.
>>>
>>>>>> +    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.
>>>
>>> Before Primary write requests are written to (nbd target), aka the Secondary
>>> disk, the orignal sector content is read from it and copied to (hidden buf
>>> disk) by drive-backup. It obviously will not overwrite the data in (active
>>> disk).
>>>
>>>>>> +    3) Primary write requests will be written to Secondary disk.
>>>
>>> Primary write requests are written to (nbd target).
>>>
>>>>>> +    4) Secondary write requests will be buffered in the Disk buffer and 
>>>>>> it
>>>>>> +       will overwrite the existing sector content in the buffer.
>>>
>>> Secondary write request will be written in (active disk) as usual.
>>>
>>> Finally, when checkpoint arrives, if you want to sync with primary, just 
>>> drop
>>> data in (hidden buf disk) and (active disk); when failover happends, if you
>>> want to promote secondary vm, you can commit (active disk) to (nbd target), 
>>> and
>>> drop data in (hidden buf disk).
>>>
>>> Fam
>>> .
>>>
>>
> .
> 




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