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

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()
Date: Tue, 29 Mar 2016 18:09:07 +0200
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On 29.03.2016 18:03, Dr. David Alan Gilbert wrote:
> * Max Reitz (address@hidden) wrote:
>> On 29.03.2016 17:54, Dr. David Alan Gilbert wrote:
>>> * 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'.
>> Because in that mode quorum successively reads from all of its children
>> and returns the first successful result. So the First successful Input
>> is the one that becomes quorum's Output (there isn't much of a
>> successive output, so it doesn't make much sense to call that the First
>> Output, though...).
>> I didn't name it, though. *waves hands defensively* :-)
> But that description doesn't make sense for what COLO uses it for.
> They have, on the primary host:
>    0) Local disk
>    1) an NBD connection to the secondary
> So in theory a read should always happen from (0) and writes should
> go to both.

Well that's the way it works, isn't it?

I didn't mention what happens with writes, but those are indeed
distributed to all of quorum's children. And as long as the local disk
doesn't fail, data is always read from it alone.

All you need to do is make sure that the local disk is the first node in
whatever order FIFO is supposed to use.


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