[Pacemaker] 2 node cluster questions

[Nick Khamis]

On Fri, Nov 25, 2011 at 7:05 AM, Hellemans Dirk D
<Dirk.Hellemans at hpcds.com> wrote:
> Hello everyone,
>
>
>
> I’ve been reading a lot lately about using Corosync/Openais in combination
> with Pacemaker: SuSe Linux documentation, Pacemaker & Linux-ha website,
> interesting blogs, mailinglists, etc. As I’m particularly interested in how
> well two node clusters (located within the same server room) are handled, I
> was a bit confused by the fact that quorum disks/ quorum servers are (not
> yet?) supported/used. Some suggested to add a third node which is not
> actively participating (e.g. only running corosync.... or with hearbeat but
> in standby mode). That might be a solution but doesn’t “feel” right,
> especially if you consider multiple two-node clusters... that would require
> a lot of extra “quorum only nodes”. Somehow SBD (storage based death) in
> combination with a hardware watchdog timer seemed to also provide a
> solution: run it on top of iSCSI storage and you end up with a fencing
> device and some sort of “network based quorum” as tiebreaker. If one node
> loses network connectivity, sbd + watchdog will make sure it’s being fenced.
>
>
>
> I’d love to hear your ideas about 2 node cluster setups. What is the best
> way to do it? Any chance we’ll get quorum disks/ quorum servers in the
> (near) future?
>
>
>
> In addition, say you’re not using sbd but an IPMI based fencing solution.
> You lose network connectivity on one of the nodes (I know, they’re redundant
> but still...sh*t happens ;) Does Pacemaker know which of both nodes lost
> network connectivity? E.g.: node 1 runs Oracle database, node 2 nothing.
> Node 2 loses network connectivity (e.g. both NICs without signal because
> unplugged by an errant technician ;) )... => split brain situation occurs,
> but who’ll be fenced? The one with Oracle running ?? I really hope not...
> cause in this case, the cluster can “see” there’s no signal on the NICs of
> node2. Would be interesting to know more about how Pacemaker/corosync makes
> such kind of decisions... how to choose which one will be fenced in case of
> split brain. Is it randomly chosen? Is it the DC which decides? Based on NIC
> state? I did some quick testing with 2 VMs and at first, it looks like
> Pacemaker/corosync always fences the correct node, or: the node where I
> unplugged the “virtual” cable.
>
>
>
> I’m curious!
>
>
>
> Thanks a lot!
>
>
>
> Best regards,
>
> Dirk
>
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>
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>
>

Hello Hellemans,

I can't speak much for the lack of quorum support outside of a 2 node
cluster, other than you can
get additioanl support using cman. I personally don't like adding
another partial cluster stack into
the equations. And I don't think Suse Enterprise supports the RH
technology. As for node fencing
order, this can always be manipulated through the use of:

* Resource stickiness
* Location/collocation/anti-location

You could also using the network RAs (IPAddr2, and ping), to determine
node network connectivity,
and *order* the way your configuration starts-up.

Cheers,

Nick.