[Pacemaker] [RFC] Automatic nodelist synchronization between corosync and pacemaker

Andrew Beekhof andrew at beekhof.net
Wed Mar 6 19:37:57 EST 2013 

On Thu, Mar 7, 2013 at 2:41 AM, Vladislav Bogdanov <bubble at hoster-ok.com> wrote:
> 06.03.2013 08:35, Andrew Beekhof wrote:

>> So basically, you want to be able to add/remove nodes from nodelist.*
>> in corosync.conf and have pacemaker automatically add/remove them from
>> itself?
>
> Not corosync.conf, but cmap which is initially (partially) filled with
> values from corosync.conf.
>
>>
>> If corosync.conf gets out of sync (admin error or maybe a node was
>> down when you updated last) they might well get added back - I assume
>> you're ok with that?
>> Because there's no real way to know the difference between "added
>> back" and "not removed from last time".
>
> Sorry, can you please reword?

When node-A comes up with "node-X" that no-one else has, the cluster
has no way to know if node-X was just added, or if the admin forgot to
remove it on node-A.

>> Or are you planning to never update the on-disk corosync.conf and only
>> modify the in-memory nodelist?
>
> That depends on the actual use case I think.
>
> Hm. Interesting, how corosync behave when new dynamic nodes are added to
> cluster... I mean following: we have static corosync.conf with nodelist
> containing f.e. 3 entries, then we add fourth entry via cmap and boot
> fourth node. What should be in corosync.conf of that node?

I don't know actually.  Try it and see if it works without the local
node being defined?

> I believe in
> wont work without that _its_ fourth entry. Ugh. If so, then no fully
> dynamic "elastic" cluster which I was dreaming of is still possible
> because out-of-the-box when using dynamic nodelist.
>
> The only way to have this I see is to have static nodelist in
> corosync.conf with all possible nodes predefined. And never edit it in
> cmap. So, my original point
> * Remove nodes from CIB when they are removed from a nodelist.
> does not fit.
>
> By elastic I mean what was discussed on corosync list when Fabio started
> with votequorum design and what then appeared in votequorum manpage:
> ===========
> allow_downscale: 1
>
> Enables allow downscale (AD) feature (default: 0).
>
> The general behaviour of votequorum is to never decrease expected votes
> or quorum.
>
> When  AD  is  enabled,  both expected votes and quorum are recalculated
> when a node leaves the cluster in a clean state (normal corosync  shut-
> down process) down to configured expected_votes.

But thats very different to removing the node completely.
You still want to know its in a sane state.

> Example use case:
>
> 1) N node cluster (where N is any value higher than 3)
> 2) expected_votes set to 3 in corosync.conf
> 3) only 3 nodes are running
> 4) admin requires to increase processing power and adds 10 nodes
> 5) internal expected_votes is automatically set to 13
> 6) minimum expected_votes is 3 (from configuration)
> - up to this point this is standard votequorum behavior -
> 7) once the work is done, admin wants to remove nodes from the cluster
> 8) using an ordered shutdown the admin can reduce the cluster size
>    automatically back to 3, but not below 3, where normal quorum
>    operation will work as usual.
> =============
>
> What I would expect from pacemaker, is to automatically remove nodes
> down to 3 at step 8 (just follow quorum) if AD is enabled AND pacemaker
> is instructed to follow that (with some other cmap switch). And also to
> reduce number of allocated clone instances. Sure, all nodes must have
> equal number of votes (1).
>
> Is it ok for you?

Not really.
We simply don't have enough information to do the removal.
All we get is "node gone", we have to do a fair bit of work to
calculate if it was clean at the time or not (and clean to corosync
doesn't always imply clean to pacemaker).

So back to the start, why do you need pacemaker to forget about the
other 10 nodes?
(because everything apart from that should already work).

>
>>
>>>
>>>>
>>>>> That
>>>>> would be OK if number of clone instances does not raise with that...
>>>>
>>>> Why?  If clone-node-max=1, then you'll never have more than the number
>>>> of active nodes - even if clone-max is greater.
>>>
>>> Active (online) or known (existing in a <nodes> section)?
>>> I've seen that as soon as node appears in <nodes> even in offline state,
>>> new clone instance is allocated.
>>
>> $num_known instances will "exist", but only $num_active will be running.
>
> Yep, that's what I say. I see them in crm_mon or 'crm status' and they
> make my life harder ;)
> That remaining instances are "allocated" but not running.
>
> I can agree that this issue is very "cosmetic" one, but its existence
> conflicts with my perfectionism so I'd like to resolve it ;)
>
>>
>>>
>>> Also, on one cluster with post-1.1.7 with openais plugin I have 16 nodes
>>> configured in totem.interface.members, but only three nodes in <nodes>
>>> CIB section, And I'm able to allocate at least 8-9 instances of clones
>>> with clone-max.
>>
>> Yes, but did you set clone-node-max?  One is the global maximum, the
>> other is the per-node maximum.
>>
>>> I believe that pacemaker does not query
>>> totem.interface.members directly with openais plugin,
>>
>> Correct.
>>
>>> and
>>> runtime.totem.pg.mrp.srp.members has only three nodes.
>>> Did that behavior change recently?
>>
>> No.
>>
>>>
>>>>
>>>>>
>>>>>
>>>>>> For node removal we do require crm_node --remove.
>>>>>>
>>>>>> Is this not sufficient?
>>>>>
>>>>> I think it would be more straight-forward if there is only one origin of
>>>>> membership information for entire cluster stack, so proposal is to
>>>>> automatically remove node from CIB when it disappears from corosync
>>>>> nodelist (due to removal by admin). That nodelist is not dynamic (read
>>>>> from a config and then may be altered with cmapctl).
>>>>
>>>> Ok, but there still needs to be a trigger.
>>>> Otherwise we waste cycles continuously polling corosync for something
>>>> that is probably never going to happen.
>>>
>>> Please see above (cmap_track_add).
>>>
>>>>
>>>> Btw. crm_node doesn't just remove the node from the cib, its existence
>>>> is preserved in a number of caches which need to be purged.
>>>
>>> That could be done in a cmap_track_add's callback function too I think.
>>>
>>>> It could be possible to have crm_node also use the CMAP API to remove
>>>> it from the running corosync, but something would still need to edit
>>>> corosync.conf
>>>
>>> Yes, that is to admin.
>>> Btw I think more about scenario Fabio explains in votequorum(8) in
>>> 'allow_downscale' section - that is the one I'm interested in.
>>>
>>>>
>>>> IIRC, pcs handles all three components (corosync.conf, CMAP, crm_node)
>>>> as well as the "add" case.
>>>
>>> Good to know. But, I'm not ready yet to switch to it.
>>>
>>>>
>>>>> Of course, it is possible to use crm_node to remove node from CIB too
>>>>> after it disappeared from corosync, but that is not as elegant as
>>>>> automatic one IMHO. And, that should not be very difficult to implement.
>>>>>
>>>>>>
>>>>>>> utilizing
>>>>>>> possibilities CMAP and votequorum provide.
>>>>>>>
>>>>>>> Idea is to:
>>>>>>> * Do not add nodes from nodelist to CIB if their join-count in cmap is
>>>>>>> zero (but do not touch CIB nodes which exist in a nodelist and have zero
>>>>>>> join-count in cmap).
>>>>>>> * Install watches on a cmap nodelist.node and
>>>>>>> runtime.totem.pg.mrp.srp.members subtrees (cmap_track_add).
>>>>>>> * Add missing nodes to CIB as soon as they are both
>>>>>>> ** defined in a nodelist
>>>>>>> ** their join count becomes non-zero.
>>>>>>> * Remove nodes from CIB when they are removed from a nodelist.
>>>>>>
>>>>>> From _a_ nodelist or _the_ (optional) corosync nodelist?
>>>>>
>>>>> From the nodelist.node subtree of CMAP tree.
>>>>>
>>>>>>
>>>>>> Because removing a node from the cluster because it shut down is... an
>>>>>> interesting idea.
>>>>>
>>>>> BTW even that could be possible if quorum.allow_downscale is enabled,
>>>>> but requires much more thinking and probably more functionality from
>>>>> corosync. I'm not ready to comment on that yet though.
>>>>
>>>> "A node left but I still have quorum" is very different to "a node
>>>> left... what node?".
>>>> Also, what happens after you fence a node... do we forget about it too?
>>>
>>> quorum.allow_downscale mandates that it is active only if node leaves
>>> the cluster in a clean state.
>>>
>>> But, from what I know, corosync does not remove node from a
>>> nodelist.node neither itself nor on request from votequorum, that's why
>>> I say about "more functionality from corosync".
>>> If votequorum could distinguish "static" node (listed in config) from
>>> "dynamic" node (added on-the-fly), and manage list of "dynamic" ones if
>>> allow_downscale is enabled, that would do the trick.
>>
>> I doubt that request would get very far, but I could be wrong.
>>
>>>
>>>>
>>>>>
>>>>> I was about node removal from a CMAP's nodelist with corosync_cmapctl
>>>>> command. Of course, absence of (optional) nodelist in CMAP would result
>>>>> in NOOP because there is no removal event on a nodelist.node tree from cmap.
>>>>>
>>>>>>
>>>>>>> Certainly, this requires some CMAP values (especially votequorum ones
>>>>>>> and may be totem mode) to have some 'well-known' values, f.e. only UDPU
>>>>>>> mode and quorum.allow_downscale=1, that should be defined yet.
>>>>>>>
>>>>>>> May be, it also have sense to make this depend on some new CMAP
>>>>>>> variable, f.e. nodelist.dynamic=1.
>>>>>>>
>>>>>>> I would even try to implement this if general agreement is gained and
>>>>>>> nobody else wants to implement this.
>>>>>>>
>>>>>>> Can you please comment on this?
>>>>>>>
>>>>>>> Vladislav
>>>
>>>
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>>
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>
>
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