On Thu, Oct 20, 2016 at 09:57:33PM +0800, zhanghailiang wrote:
> COLO block replication doesn't support the shared disk case,
> Here we try to implement it.
> 
> Just as the scenario of non-shared disk block replication,
> we are going to implement block replication from many basic
> blocks that are already in QEMU.
> The architecture is:
> 
>          virtio-blk                     ||                               .----------
>              /                          ||                               | Secondary
>             /                           ||                               '----------
>            /                            ||                                 virtio-blk
>           /                             ||                                      |
>           |                             ||                               replication(5)
>           |                    NBD  -------->   NBD   (2)                       |
>           |                  client     ||    server ---> hidden disk <-- active disk(4)
>           |                     ^       ||                      |
>           |              replication(1) ||                      |
>           |                     |       ||                      |
>           |   +-----------------'       ||                      |
>          (3)  |drive-backup sync=none   ||                      |
> --------. |   +-----------------+       ||                      |
> Primary | |                     |       ||           backing    |
> --------' |                     |       ||                      |
>           V                     |                               |
>        +-------------------------------------------+            |
>        |               shared disk                 | <----------+
>        +-------------------------------------------+
> 1) Primary writes will read original data and forward it to Secondary
>    QEMU.
> 2) The hidden-disk will buffers the original content that is modified
>    by the primary VM. It should also be an empty disk, and
>    the driver supports bdrv_make_empty() and backing file.
> 3) Primary write requests will be written to Shared disk.
> 4) Secondary write requests will be buffered in the active disk and it
>    will overwrite the existing sector content in the buffe

This design looks good.  I have not reviewed the patches in detail but
will review the next revision.

Stefan