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

From: Alberto Garcia
Subject: Re: [Qemu-devel] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()
Date: Wed, 30 Mar 2016 13:39:01 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Tue 29 Mar 2016 05:51:22 PM CEST, Max Reitz wrote:
>> 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.
> However, the order should be queriable after the fact, and there are
> three ways I see to accomplish this:
> (1) Make this information queriable as driver-specific BDS information.
>     I personally don't like it very much, but it would be fine.
> (2) Implement query-block-node-tree, make the order of child nodes
>     significant and thus represent the FIFO order there. I don't like
>     this because it would mean returning two orders through that child
>     node list: One is the numeric order (children.0, children.1, ...)
>     and another is the FIFO order, which are not necessarily equal.
> (3) Fix FIFO order to the child name (its role). I'm very much in favor
>     of this.
> While I don't have good arguments against (1), I think I have good
> arguments for (3) instead: It just doesn't make sense to have a numeric
> order of children if this order doesn't mean anything; especially if you
> suddenly do need the list of child nodes to be ordered. To me, it
> doesn't make any sense to introduce a new hidden order which takes
> precedence over this obvious user-visible order.

I'm not sure if I understand correctly what you mean in (3). The
user-visible FIFO order is the one specified when the Quorum is created:


Would you then call those BDS children.0, children.1, etc and make those
names be the ones that actually define how they are ordered internally?

I also have another (not directly related) question: why not simply use
the node name when removing children? I understood that the idea was
that it's possible to have the same node attached twice to the same
Quorum, but can you actually do that? And what's the use case?


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