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

[Fam Zheng <famz@redhat.com>]

On Wed, 02/25 17:10, 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.
> > 
> > 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 >>>>
> 
> Why nbd target has backing image ever?

It's not strictly necessary, depending on your VM disk configuration. (for
example at the time of vm booting, your image already points to a backing file,
etc.

Fam