Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication

[Stefan Hajnoczi]

On Thu, Oct 15, 2015 at 10:19:17AM +0800, Wen Congyang wrote:
> On 10/14/2015 10:27 PM, Stefan Hajnoczi wrote:
> > On Tue, Oct 13, 2015 at 05:08:17PM +0800, Wen Congyang wrote:
> >> On 10/13/2015 12:27 AM, Stefan Hajnoczi wrote:
> >>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote:
> >>>> +        /* start backup job now */
> >>>> +        bdrv_op_unblock(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET,
> >>>> +                        s->active_disk->backing_blocker);
> >>>> +        bdrv_op_unblock(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE,
> >>>> +                        s->hidden_disk->backing_blocker);
> >>>
> >>> Why is it safe to unblock these operations?
> >>>
> >>> Why do they have to be blocked for non-replication users?
> >>
> >> hidden_disk and secondary disk are opened as backing file, so it is 
> >> blocked for
> >> non-replication users.
> >> What can I do if I don't unblock it and want to do backup?
> > 
> > CCing Jeff Cody, block jobs maintainer
> > 
> > You need to explain why it is safe remove this protection.  We can't
> > merge code that may be unsafe.
> > 
> > I think we can investigate further by asking: when does QEMU code assume
> > the backing file is read-only?
> 
> The backing file is opened in read-only mode. I want to reopen it in 
> read-write
> mode here in the next version(So the patch 1 will be dropped)
> 
> > 
> > I haven't checked but these cases come to mind:
> > 
> > Operations that move data between BDS in the backing chain (e.g. commit
> > and stream block jobs) will lose or overwrite data if the backing file
> > is being written to by another coroutine.
> > 
> > We need to prevent users from running these operations at the same time.
> 
> Yes, but qemu doesn't provide such API.

This series can't be merged unless it is safe.

Have you looked at op blockers and thought about how to prevent unsafe
operations?

Stefan