[ClusterLabs] Two nodes cluster issue
Ken Gaillot
kgaillot at redhat.com
Mon Jul 24 18:04:38 EDT 2017
On Mon, 2017-07-24 at 21:29 +0000, Tomer Azran wrote:
> I tend to agree with Klaus – I don't think that having a hook that
> bypass stonith is the right way. It is better to not use stonith at
> all.
>
> I think I will try to use an iScsi target on my qdevice and set SBD to
> use it.
Certainly, two levels of real stonith is best -- but (at the risk of
committing heresy) I can see Shashank's point.
If you've got extensive redundancy everywhere else, then the chances of
something taking out both the node and its IPMI, yet still allowing it
to interfere with shared resources, is very small. It comes down to
whether you're willing to accept that small risk. Such a setup is
definitely better than disabling fencing altogether, because the IPMI
fence level safely handles all node failure scenarios that don't also
take out the IPMI (and the bypass actually handles a complete power cut
safely).
If you can do a second level of real fencing, that is of course
preferred.
> I still don't understand why qdevice can't take the place SBD with
> shared storage; correct me if I'm wrong, but it looks like both of
> them are there for the same reason.
>
>
>
> From: Klaus Wenninger [mailto:kwenning at redhat.com]
> Sent: Monday, July 24, 2017 9:01 PM
> To: Cluster Labs - All topics related to open-source clustering
> welcomed <users at clusterlabs.org>; Prasad, Shashank <ssprasad at vanu.com>
> Subject: Re: [ClusterLabs] Two nodes cluster issue
>
>
>
>
> On 07/24/2017 07:32 PM, Prasad, Shashank wrote:
>
>
> Sometimes IPMI fence devices use shared power of the node, and
> it cannot be avoided.
>
> In such scenarios the HA cluster is NOT able to handle the
> power failure of a node, since the power is shared with its
> own fence device.
>
> The failure of IPMI based fencing can also exist due to other
> reasons also.
>
>
>
> A failure to fence the failed node will cause cluster to be
> marked UNCLEAN.
>
> To get over it, the following command needs to be invoked on
> the surviving node.
>
>
>
> pcs stonith confirm <failed_node_name> --force
>
>
>
> This can be automated by hooking a recovery script, when the
> the Stonith resource ‘Timed Out’ event.
>
> To be more specific, the Pacemaker Alerts can be used for
> watch for Stonith timeouts and failures.
>
> In that script, all that’s essentially to be executed is the
> aforementioned command.
>
>
>
> If I get you right here you can disable fencing then in the first
> place.
> Actually quorum-based-watchdog-fencing is the way to do this in a
> safe manner. This of course assumes you have a proper source for
> quorum in your 2-node-setup with e.g. qdevice or using a shared
> disk with sbd (not directly pacemaker quorum here but similar thing
> handled inside sbd).
>
>
>
>
> Since the alerts are issued from ‘hacluster’ login, sudo
> permissions for ‘hacluster’ needs to be configured.
>
>
>
> Thanx.
>
>
>
>
>
> From: Klaus Wenninger [mailto:kwenning at redhat.com]
> Sent: Monday, July 24, 2017 9:24 PM
> To: Kristián Feldsam; Cluster Labs - All topics related to
> open-source clustering welcomed
> Subject: Re: [ClusterLabs] Two nodes cluster issue
>
>
>
>
> On 07/24/2017 05:37 PM, Kristián Feldsam wrote:
>
>
> I personally think that power off node by switched pdu
> is more safe, or not?
>
>
>
> True if that is working in you environment. If you can't do a
> physical setup
> where you aren't simultaneously loosing connection to both
> your node and
> the switch-device (or you just want to cover cases where that
> happens)
> you have to come up with something else.
>
>
>
>
>
>
> S pozdravem Kristián Feldsam
> Tel.: +420 773 303 353, +421 944 137 535
> E-mail.: support at feldhost.cz
>
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>
>
>
>
> On 24 Jul 2017, at 17:27, Klaus Wenninger
> <kwenning at redhat.com> wrote:
>
>
>
>
> On 07/24/2017 05:15 PM, Tomer Azran wrote:
>
>
> I still don't understand why the qdevice
> concept doesn't help on this situation. Since
> the master node is down, I would expect the
> quorum to declare it as dead.
>
>
> Why doesn't it happens?
>
>
>
>
> That is not how quorum works. It just limits the
> decision-making to the quorate subset of the cluster.
> Still the unknown nodes are not sure to be down.
> That is why I suggested to have quorum-based
> watchdog-fencing with sbd.
> That would assure that within a certain time all nodes
> of the non-quorate part
> of the cluster are down.
>
>
>
>
>
>
>
>
> On Mon, Jul 24, 2017 at 4:15 PM +0300, "Dmitri
> Maziuk" <dmitri.maziuk at gmail.com> wrote:
>
> On 2017-07-24 07:51, Tomer Azran wrote:
> > We don't have the ability to use it.
> > Is that the only solution?
>
> No, but I'd recommend thinking about it first. Are you sure you will
> care about your cluster working when your server room is on fire? 'Cause
> unless you have halon suppression, your server room is a complete
> write-off anyway. (Think water from sprinklers hitting rich chunky volts
> in the servers.)
>
> Dima
>
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> --
> Klaus Wenninger
>
> Senior Software Engineer, EMEA ENG Openstack Infrastructure
>
> Red Hat
>
> kwenning at redhat.com
>
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--
Ken Gaillot <kgaillot at redhat.com>
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