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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()
Date: Tue, 29 Mar 2016 09:44:04 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

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.

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

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