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

[Wen Congyang]

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?

Thanks
Wen Congyang

> 
> 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
> .
>