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

[Alberto Garcia]

On Wed 30 Mar 2016 05:07:15 PM CEST, Max Reitz wrote:
>> 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?
>
> What I like about using the child role name is that it automatically
> prevents you from specifying a node that is not a child of the given
> parent.

Right, but checking if a node is not a child and returning an error is
very simple. And it doesn't require the user to keep track of the node
name *and* the child role name.

Unless I'm forgetting something this would be the first time we expose
the child role name in the API, that's why I'm wondering if it's
something worth doing.

> Which makes me notice that it might be a good idea to require the user
> to specify the child's role when adding a new child. In this version
> of this series (where only quorum is supported), the children are just
> inserted in numerical order (first free slot is taken first), but
> maybe the user wants to insert them in a different order.

For the Quorum case it totally makes sense to let the user choose the
position of the new child.

But for creating a Quorum array in the first place we don't require
that, the order is the one that the user provides, and the user does not
need to know about the child role names at that point.

> And the "filling up quorum's children" topic then makes me notice that
> (x-)blockdev-change should probably be transaction-able (so you can
> restructure the whole BDS graph in a single transaction), but that's
> something we can care about later on.

I agree.

Berto