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

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()
Date: Tue, 29 Mar 2016 16:50:25 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

* 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


> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org

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

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