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

[Wen Congyang]

On 02/25/2015 04:58 PM, Fam Zheng wrote:
> On Wed, 02/25 16:36, 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 :)
>>
>> Does hidden target is only used for COLO?
>>
> 
> I'm not sure I get your question.
> 
> In this case yes, this is a dedicate target that's only written to by COLO's
> secondary VM.
> 
> In other general cases, this infrastructure could also be used for backup or
> image fleecing.

In COLO case, we can create (hidden buf disk) when starting block replication.
In other general cases, I don't know when to create (hidden buf disk)?

Thanks
Wen Congyang

> 
> Fam
> 
>>
>>>
>>> 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
>>>>> .
>>>>>
>>>>
>>> .
>>>
>>
>>
> .
>