[Pacemaker] Creating a safe cluster-node shutdown script (for when UPS goes OnBattery+LowBattery)

Thu Jul 10 19:30:35 EDT 2014

> Date: Thu, 10 Jul 2014 10:24:49 +0200
> From: tojeline at redhat.com
> To: pacemaker at oss.clusterlabs.org
> Subject: Re: [Pacemaker] Creating a safe cluster-node shutdown script (for when UPS goes OnBattery+LowBattery)
> 
> Dne 10.7.2014 03:17, Giuseppe Ragusa napsal(a):
> > On Thu, Jul 10, 2014, at 00:06, Andrew Beekhof wrote:
> >>
> >> On 9 Jul 2014, at 10:28 pm, Giuseppe Ragusa <giuseppe.ragusa at hotmail.com> wrote:
> >>
> >>> On Tue, Jul 8, 2014, at 02:59, Andrew Beekhof wrote:
> >>>>
> >>>> On 4 Jul 2014, at 3:16 pm, Giuseppe Ragusa <giuseppe.ragusa at hotmail.com> wrote:
> >>>>
> >>>>> Hi all,
> >>>>> I'm trying to create a script as per subject (on CentOS 6.5, CMAN+Pacemaker, only DRBD+KVM active/passive resources; SNMP-UPS monitored by NUT).
> >>>>>
> >>>>> Ideally I think that each node should stop (disable) all locally-running VirtualDomain resources (doing so cleanly demotes than downs the DRBD resources underneath), then put itself in standby and finally shutdown.
> >>>>
> >>>> Since the end goal is shutdown, why not just run 'pcs cluster stop' ?
> >>>
> >>> I thought that this action would cause communication interruption (since Corosync would be not responding to the peer) and so cause the other node to stonith us;
> >>
> >> No. Shutdown is a globally co-ordinated process.
> >> We don't fence nodes we know shut down cleanly.
> >
> > Thanks for the clarification.
> > Now that you said it, it seems also logical and even obvious ;>
> >
> >>> I know that ideally the other node too should perform "pcs cluster stop" in short, since the same UPS powers both, but I worry about timing issues (and "races") in UPS monitoring since it is a large Enterprise UPS monitored by SNMP.
> >>>
> >>> Furthermore I do not know what happens to running resources at "pcs cluster stop": I infer from your suggestion that resources are brought down and not migrated on the other node, correct?
> >>
> >> If the other node is shutting down too, they'll simply be stopped.
> >> Otherwise we'll try to move them.
> >
> > It's the "moving" that worries me :)
> >
> >>>> Possibly with 'pcs cluster standby' first if you're worried that stopping the resources might take too long.
> >
> > I forgot to ask: in which way would a previous standby make the resources stop sooner?
> >
> >>> I thought that "pcs cluster standby" would usually migrate the resources to the other node (I actually tried it and confirmed the expected behaviour); so this would risk to become a race with the timing of the other node standby,
> >>
> >> Not really, at the point the second node runs 'standby' we'll stop trying to migrate services and just stop them everywhere.
> >> Again, this is a centrally controlled process, timing isn't a problem.
> >
> > I understand that, "eventually", timing won't be a problem and resources will "eventually" stop, but from your description I'm afraid that some delaying could result in the total shutdown process, arising from possibly unsynchronized UPS notifications on the nodes (first node starts standby, resources start to move, THEN second node starts standby).
> >
> > So now I'm taking your advice and I'll modify the script to user cluster stop but, with the aim of avoiding the aforementioned delay (if it actually represents a possibility), I would like to ask you three questions:
> >
> Hi,
> "pcs cluster stop --all" does not work on 6.5 with pcs-0.9.90 which I 
> believe is shipped with 6.5. You need to install current pcs version 
> from https://github.com/feist/pcs and run pcsd service on all nodes.

Hi,
thanks for the information.
>From reading the discussion under various Pacemaker-related bugzilla entries (on Red Hat bugzilla) I'm suspecting that the package will be rebased/updated soon (possibly for RHEL 6.6), so I could either wait or temporarily recompile/create an updated package myself.

> > *) if I simply issue a "pcs cluster stop --all" from the first node that gets notified of UPS critical status, do I risk any adverse effect when the other node asynchronously gives the same command some time later (before/after the whole cluster stop sequence completes)?
> It just runs "service pacemaker stop" and "service cman stop" on every 
> node. It should have no effect once the services are already stopped.

Ok for the absence of adverse effects, but the error detection could be "difficult" (see your warning on exit codes below):
it could have exit code != 0 if the other node was already powered off or otherwise inoperative, right?
And what would be the exit code if the cluster is already off on this node (because the other one caught the UPS signal first)? I suppose that would be an exit code != 0 too.

> > *) does the aforementioned "pcs cluster stop --all" command return only after the cluster stop sequence has actually/completely ended (so as to safely issue a "shutdown -h now" immediately afterwards)?
> Yes. You need to check the return code, non-zero return code means some 
> error has occurred and some nodes haven't been stopped.
> When you shutdown one node and then try to run "pcs cluster stop --all" 
> on the other one it will fail and return non-zero return code (obviously).

I suppose I could simply log the exit code and assume that everything will go reasonably well anyway, but all the possible different conditions/results make me a bit nervous when thinking of a logic/algorithm to use and remind me of the hint that Digimer gave at the beginning of this thread: beware of bugs when developing a script for such a delicate purpose (programmatically bringing down an otherwise healthy cluster).

> > *) is the "pcs cluster stop --all" command known to work reliably on current CentOS 6.5? (I ask since I found some discussion around "pcs cluster start" related bugs)
> See above.

So I'm thinking of a different strategy in order to stick to the advice of keeping things simple and straight to the point, so these are my conclusions:

*) I'll get  rid of my too elaborate/peculiar loop, detections, timeouts

*) no "pcs cluster stop" (with or without --all) since it does not meet my goal (i.e. bring resources, cluster and then nodes down with the absolute minimum fuss and resources ping-pong should the nodes catch the UPS status with some delay between them)

*) no "pcs cluster standby" for the same reason given above

*) the cluster coming up fully working by itself at the eventual restart (power restored and UPS battery at least minimally recharged) would be desirable, but I will set for a little manual intervention once power stability has been assured by humans ;>

So this is the strategy I came up with after pondering all your advices/suggestions/explanations:

"pcs property set maintenance-mode=true" given by each node unconditionally (I think this should work when given again by the other node, independently of any delay the other node has in starting this shutdown-by-UPS procedure) followed (maybe with a little fixed sleep) by a simple local "shutdown -h +0"

given that all the resources (unmanaged, at this point) that I'm using have an initscript counterpart which is registered for system halt/reboot runlevels (but obviously not for system boot), it follows that they should come down cleanly as if they were not cluster-controlled

at the eventual restart (power restored and UPS battery at least minimally recharged) an interactively issued "pcs property unset maintenance-mode" should bring everything online again (since pacemaker and cman start automatically at system boot)

What do you think? Is this correct/sensible?

Many thanks again for all your suggestions/informations.

Regards,
Giuseppe

> Regards,
> Tomas
> >
> > Many thanks again for your invaluable help and insight.
> >
> > Regards,
> > Giuseppe
> >
> >>> so this is why I took the hassle of explicitly and orderly stopping all locally-running resources in my script BEFORE putting the local node in standby.
> >>>
> >>>> Pacemaker will stop everything in the required order and stop the node when done... problem solved?
> >>>
> >>> I thought that after a "pcs cluster standby" a regular "shutdown -h" of the operating system would cleanly bring down the cluster too,
> >>
> >> It should do
> >>
> >>> without the need for a "pcs cluster stop", given that both Pacemaker and CMAN are correctly configured for automatic startup/shutdown as operating system services (SysV initscripts controlled by CentOS 6.5 Upstart, in my case).
> >>>
> >>> Many thanks again for your always thought-provoking and informative answers!
> >>>
> >>> Regards,
> >>> Giuseppe
> >>>
> >>>>>
> >>>>> On further startup, manual intervention would be required to unstandby all nodes and enable resources (nodes already in standby and resources already disabled before blackout should be manually distinguished).
> >>>>>
> >>>>> Is this strategy conceptually safe?
> >>>>>
> >>>>> Unfortunately, various searches have turned out no "prior art" :)
> >>>>>
> >>>>> This is my tentative script (consider it in the public domain):
> >>>>>
> >>>>> ------------------------------------------------------------------------------------------------------------------------------------
> >>>>> #!/bin/bash
> >>>>>
> >>>>> # Note: "pcs cluster status" still has a small bug vs. CMAN-controlled Corosync and would always return != 0
> >>>>> pcs status > /dev/null 2>&1
> >>>>> STATUS=$?
> >>>>>
> >>>>> # Detect if cluster is running at all on local node
> >>>>> # TODO: detect node already in standby and bypass this
> >>>>> if [ "${STATUS}" = 0 ]; then
> >>>>>     local_node="$(cman_tool status | grep -i 'Node[[:space:]]*name:' | sed -e 's/^.*Node\s*name:\s*\([^[:space:]]*\).*$/\1/i')"
> >>>>>     for local_resource in $(pcs status 2>/dev/null | grep "ocf::heartbeat:VirtualDomain.*${local_node}\\s*\$" | awk '{print $1}'); do
> >>>>>         pcs resource disable "${local_resource}"
> >>>>>     done
> >>>>>     # TODO: each resource disabling above may return without waiting for complete stop - wait here for "no more resources active"? (but avoid endless loops)
> >>>>>     pcs cluster standby "${local_node}"
> >>>>> fi
> >>>>>
> >>>>> # Shut down gracefully anyway at the end
> >>>>> /sbin/shutdown -h +0
> >>>>>
> >>>>> ------------------------------------------------------------------------------------------------------------------------------------
> >>>>>
> >>>>> Comments/suggestions/improvements are more than welcome.
> >>>>>
> >>>>> Many thanks in advance.
> >>>>>
> >>>>> Regards,
> >>>>> Giuseppe
> >>>>>
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> >>> --
> >>>   Giuseppe Ragusa
> >>>   giuseppe.ragusa at fastmail.fm
> >>>
> >>>
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