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

[Fam Zheng]

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

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.

Fam