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

[Dr. David Alan Gilbert]

* Max Reitz (address@hidden) wrote:
> On 29.03.2016 17:50, Dr. David Alan Gilbert wrote:
> > * Eric Blake (address@hidden) wrote:
> >> On 03/29/2016 09:38 AM, Max Reitz wrote:
> >>> On 17.03.2016 10:56, Wen Congyang wrote:
> >>>> On 03/17/2016 05:48 PM, Dr. David Alan Gilbert wrote:
> >>>
> >>> [...]
> >>>
> >>>>> The children.0 notation is really confusing in the way that Berto
> >>>>> describes; I hit this a couple of months ago and it really doesn't
> >>>>> make sense.
> >>>>
> >>>> Do you mean: read from children.1 first, and then read from children.0 in
> >>>> fifo mode? Yes, the behavior is very strange.
> >>>
> >>> So is this intended or is it not? In
> >>> http://lists.nongnu.org/archive/html/qemu-block/2016-03/msg00526.html
> >>> you said that it is.
> >>>
> >>> I myself would indeed say it is very strange. If I were a user, I would
> >>> not expect this behavior. And as I developer, I think that how a BDS's
> >>> child is used by its parent should solely depend on its role (e.g.
> >>> whether it is "children.0" or "children.1").
> >>
> >> It sounds like the argument here, and in Max's thread on
> >> query-block-node-tree, is that we DO have cases where order matters, and
> >> so we need a way for the hot-add operation to explicitly specify where
> >> in the list a child is inserted (whether it is being inserted as the new
> >> primary image, or explicitly as the last resort, or somewhere in the
> >> middle).  An optional parameter, that defaults to appending, may be ok,
> >> but we definitely need to consider how the order of children is affected
> >> by hot-add.
> > 
> > Certainly in the COLO case the two children are not identical; and IMHO we 
> > need
> > to get away from thinking about ordering and start thinking about functional
> > namingd - children.0/children.1 doesn't suggest the fact they behave
> > differently.
> 
> To me it does. If quorum is operating in a mode call "FIFO" I would
> expect some order on the child nodes, and if the child nodes are
> actually numbered in an ascending order, that is an obvious order.

I don't understand why it's called 'FIFO'.

Dave

> Max
> 



--
Dr. David Alan Gilbert / address@hidden / Manchester, UK