[Pacemaker] pacemaker/dlm problems

[Andrew Beekhof]

On Tue, Sep 27, 2011 at 5:07 PM, Vladislav Bogdanov
<bubble at hoster-ok.com> wrote:
> 27.09.2011 08:59, Andrew Beekhof wrote:
> [snip]
>>>>>>> I agree with Jiaju
>>>>>>> (https://lists.linux-foundation.org/pipermail/openais/2011-September/016713.html),
>>>>>>> that could be solely pacemaker problem, because it probably should
>>>>>>> originate fencing itself is such situation I think.
>>>>>>>
>>>>>>> So, using pacemaker/dlm with openais stack is currently risky due to
>>>>>>> possible hangs of dlm_lockspaces.
>>>>>>
>>>>>> It shouldn't be, failing to connect to attrd is very unusual.
>>>>>
>>>>> By the way, one of underlying problems, which actually made me to notice
>>>>> all this, is that pacemaker cluster does not fence its DC if it leaves
>>>>> the cluster for a very short time. That is what Jiaju told in his notes.
>>>>> And I can confirm that.
>>>>
>>>> Thats highly surprising.  Do the logs you sent display this behaviour?
>>>
>>> They do. Rest of the cluster begins the election, but then accepts
>>> returned DC back (I write this from memory, I looked at logs Sep 5-6, so
>>> I may mix up something).
>>
>> Actually, this might be possible - if DC.old came back before DC.new
>> had a chance to get elected, run the PE and initiate fencing, then
>> there would be no need to fence.
>>
>
> (text below is for pacemaker on top of openais stack, not for cman)
>
> Except dlm lockspaces are in kern_stop state, so a whole dlm-related
> part is frozen :( - clvmd in my case, but I expect the same from gfs2
> and ocfs2.
> And fencing requests originated on CPG NODEDOWN event by dlm_controld
> (with my patch to dlm_controld and your patch for
> crm_terminate_member_common()) on a quorate partition are lost. DC.old
> doesn't accept CIB updates from other nodes, so that fencing requests
> are discarded.

All the more reason to start using the stonith api directly.
I was playing around list night with the dlm_controld.pcmk code:
   https://github.com/beekhof/dlm/commit/9f890a36f6844c2a0567aea0a0e29cc47b01b787

>
> I think that problem is that membership changes are handled in a
> non-transactional way (?).

Sounds more like the dlm/etc is being dumb - if the host is back and
healthy, why would we want to shoot it?

> If pacemaker fully finish processing of one membership change - elect
> new DC on a quorate partition, and do not try to take over dc role (or
> release it) on a non-quorate partition if quorate one exists, that
> problem could be gone.

Non quorate partitions still have a DC.
They're just not supposed to do anything (depending on the value of
no-quorum-policy).

> I didn't dig into code so much, so all above is just my deduction which
> may be completely wrong.
> And of course real logic could (should) be much more complicated, with
> handling of just rebooted members, etc.
>
> (end of openais specific part)
>
>>> [snip]
>>>>>>> Although it took 25 seconds instead of 3 to break the cluster (I
>>>>>>> understand, this is almost impossible to load host so much, but
>>>>>>> anyways), then I got a real nightmare: two nodes of 3-node cluster had
>>>>>>> cman stopped (and pacemaker too because of cman connection loss) - they
>>>>>>> asked to kick_node_from_cluster() for each other, and that succeeded.
>>>>>>> But fencing didn't happen (I still need to look why, but this is cman
>>>>>>> specific).
>>>
>>> Btw this part is tricky for me to understand the underlying logic:
>>> * cman just stops cman processes on remote nodes, disregarding the
>>> quorum. I hope that could be fixed in corosync If I understand one of
>>> latest threads there right.
>>> * But cman does not do fencing of that nodes, and they still run
>>> resources. And this could be extremely dangerous under some
>>> circumstances. And cman does not do fencing even if it has fence devices
>>> configure in cluster.conf (I verified that).
>>>
>>>>>>> Remaining node had pacemaker hanged, it doesn't even
>>>>>>> notice cluster infrastructure change, down nodes were listed as a
>>>>>>> online, one of them was a DC, all resources are marked as started on all
>>>>>>> (down too) nodes. No log entries from pacemaker at all.
>>>>>>
>>>>>> Well I can't see any logs from anyone to its hard for me to comment.
>>>>>
>>>>> Logs are sent privately.
>>>>>
>>>>>>
>>>
>>> Vladislav
>>>
>>>
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>>
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>
>
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