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

[Andrew Beekhof]

On Tue, Feb 26, 2013 at 7:36 PM, Vladislav Bogdanov
<bubble at hoster-ok.com> wrote:
> 26.02.2013 11:10, Andrew Beekhof wrote:
>> On Mon, Feb 18, 2013 at 6:18 PM, Vladislav Bogdanov
>> <bubble at hoster-ok.com> wrote:
>>> Hi Andrew, all,
>>>
>>> I had an idea last night, that it may be worth implementing
>>> fully-dynamic cluster resize support in pacemaker,
>>
>> We already support nodes being added on the fly.  As soon as they show
>> up in the membership we add them to the cib.
>
> Membership (runtime.totem.pg.mrp.srp.members) or nodelist (nodelist.node)?

To my knowledge, only one (first) gets updated at runtime.
Even if nodelist.node could be updated dynamically, we'd have to poll
or be prompted to find out.
>
> I recall that when I migrated from corosync 1.4 to 2.0 (somewhere near
> pacemaker 1.1.8 release time) and replaced old-style UDPU member list
> with nodelist.node, I saw all nodes configured in that nodelist appeared
> in a CIB. For me that was a regression, because with old-style config
> (and corosync 1.4) CIB contained only nodes seen online (4 of 16).

That was a loophole that only worked when the entire cluster had been
down and the <nodes> section was empty.
People filed bugs explicitly asking for that loophole to be closed
because it was inconsistent with what the cluster did on every
subsequent startup.

> 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.

>
>
>> 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.

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.
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

IIRC, pcs handles all three components (corosync.conf, CMAP, crm_node)
as well as the "add" case.

> 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?

>
> 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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