Re: [Qemu-devel] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Wen Congyang (wency@cn.fujitsu.com) wrote:
> On 03/17/2016 07:25 PM, Dr. David Alan Gilbert wrote:
> > * Wen Congyang (wency@cn.fujitsu.com) wrote:
> >> On 03/17/2016 06:07 PM, Alberto Garcia wrote:
> >>> On Thu 17 Mar 2016 10:56:09 AM CET, Wen Congyang wrote:
> >>>>> We should have the failure modes documented, and how you'll use it
> >>>>> after failover etc Without that it's really difficult to tell if this
> >>>>> naming is right.
> >>>>
> >>>> For COLO, children.0 is the real disk, children.1 is replication
> >>>> driver.  After failure, children.1 will be removed by the user. If we
> >>>> want to continue do COLO, we need add a new children.1 again.
> >>>
> >>> What if children.0 fails ?
> >>
> >> For COLO, reading from children.1 always fails. if children.0 fails, it
> >> means that reading from the disk fails. The guest vm will see the I/O error.
> > 
> > How do we get that to cause a fail over before the guest detects it?
> > If the primary's local disk (children.0) fails then if we can failover
> > at that point then the guest carries running on the secondary without
> > ever knowing about the failure.
> 
> COLO is not designed for such case. The children.0 can also be quorum, so
> you can add more than one real disk, and get more reliability. Another
> choice is that, the real disk is an external storage, and it has
> its own replication solution.
> 
> COLO is designed for such case: the host is crashed, and the guest is still
> alive after failover, the client doesn't know this event.

That seems an odd limitation; the only thing needed for COLO to survive a disk
failure on the primary would be to ensure that the primary fails/triggers failover
if access to the local disk fails and to do it before the IO result is returned to
the guest.

Dave

> 
> Thanks
> Wen Congyang
> 
> > 
> > Dave
> > 
> >>
> >> Thanks
> >> Wen Congyang
> >>
> >>>
> >>> Berto
> >>>
> >>>
> >>> .
> >>>
> >>
> >>
> >>
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> > 
> > .
> > 
> 
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK