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From: | Changlong Xie |
Subject: | Re: [Qemu-block] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child() |
Date: | Mon, 11 Apr 2016 13:18:34 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
On 03/30/2016 11:07 PM, Max Reitz wrote:
On 30.03.2016 13:39, Alberto Garcia wrote: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: children.0.file.filename=hd0.qcow2, children.1.file.filename=hd1.qcow2, ... Would you then call those BDS children.0, children.1, etcThey are already called that way; it's not their node name but their "child role" name.and make those names be the ones that actually define how they are ordered internally?Yes, that's what I meant.
Hi Max I think you just mean what i draw in below chart: 1) Insert 4 children. 0 1 2 3 +---------------------------------------------------- |children.0|children.1|children.2|children.3| +---------------------------------------------------- 2) Remove the 2th child (s->children[1]){ "execute": "x-blockdev-change",
"arguments": { "parent": "xxx",
"child": "children.1" } } 0 1 2 3 +---------------------------------------------------- |children.0|children.1|children.2|children.3| +------------------------+------------+-------------- | | +------+ +--------+ 0 1 | 2 | +----------------v----------v------------------------ |children.0|children.1|children.2| +----------------------------------------------------Remove children.1 and shorten the array, then rename children.{2,3} to children.{1.2}
3) Insert a new child 0 1 2 3 +---------------------------------------------------- |children.0|children.1|children.2|children.3| +----------------------------------------------------But as Wen said: http://lists.nongnu.org/archive/html/qemu-devel/2016-04/msg00898.html
Everytime we try to remove a children.i (i < n-1, so it's not the last element of the array[n]), we have to rename children.{i+1, n-1} to children.{i, n-2}.
Thanks -Xie
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. 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 quorum, this is basically irrelevant if the order doesn't mean anything (which I don't like), but it may be relevant for other block drivers. 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. Max
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