Re: [Qemu-devel] [PATCH v3 5/6] replication: Implement block replication for shared disk case

[Hailiang Zhang]

Hi Stefan,

Sorry for the delayed reply.

On 2017/2/28 1:37, Stefan Hajnoczi wrote:
> On Fri, Jan 20, 2017 at 11:47:59AM +0800, zhanghailiang wrote:
> > 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 is created automatically. It 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 buffer.
> >
> > Signed-off-by: zhanghailiang <address@hidden>
> > Signed-off-by: Wen Congyang <address@hidden>
> > Signed-off-by: Zhang Chen <address@hidden>
>
> Are there any restrictions on the shared disk?  For example the -drive
> cache= mode must be 'none'.  If the cache mode isn't 'none' the
> secondary host might have old data in the host page cache.  The

While do checkpoint, we will call vm_stop(), in which, the bdrv_flush_all()
will be called, is it enough ?

> Secondary QEMU would have an inconsistent view of the shared disk.
>
> Are image file formats like qcow2 supported for the shared disk?  Extra

In the above scenario, it has no limitation of formats for the shared disk.

> steps are required to achieve consistency, see bdrv_invalidate_cache().

Hmm, in that case, we should call bdrv_invalidate_cache_all() while checkpoint.


Thanks,
Hailiang

> Stefan