Re: [Qemu-devel] [PATCH 18/18] iotests: Add test for active mirroring

[Fam Zheng]

On Mon, 09/18 18:53, Max Reitz wrote:
> >> +
> >> +        if sync_source_and_target:
> >> +            # If source and target should be in sync after the mirror,
> >> +            # we have to flush before completion
> > 
> > Not sure I understand this requirements, does it apply to libvirt and user 
> > too?
> > I.e. it's a part of the interface ? Why cannot mirror_complete do it
> > automatically?
> 
> Well, it seems to pass without this flush, but the original intention
> was this: When mirror is completed, the source node is replaced by the
> target.  All further writes are then only executed on the (former)
> target node.  So what might happen (or at least I think it could) is
> that qemu-io submits some writes, but before they are actually
> performed, the mirror block job is completed and the source is replaced
> by the target.  Then, the write operations are performed on the target
> but no longer on the source, so source and target are then out of sync.
> For the mirror block job, that is fine -- at the point of completion,
> source and target were in sync.  The job doesn't care that they get out
> of sync after completion.  But here, we have to care or we can't compare
> source and target.
> 
> The reason for why it always seems to pass without a flush is that every
> submitted write is actually sent to the mirror node before it yields for
> the first time.  But I wouldn't bet on that, so I think it's better to
> keep the flush before completing the block job.

OK, that makes sense. Thanks.

Fam