[Pacemaker] 2 node cluster questions

Wed Nov 30 14:05:42 UTC 2011

Hi Mark,

Thanks for your help! Indeed, you get at race condition... that’s why an external quorum daemon (such as the one supplied with HP’s ServiceGuard) would be nice. It looks like this is what Linux HA is heading for (http://theclusterguy.clusterlabs.org/post/907043024/introducing-the-pacemaker-master-control-process-for, setup number 4) but it’s not there yet.

The only way to do it with SBD is to avoid auto startup of a cluster node (e.g. disable corosync and pacemaker init scripts) --> avoids fencing the other after being fenced. Or to use SBD on iSCSI storage... if you have no network connection, you cannot fence and sdb will make watchdog timeout --> the node which lost network connectivity is going to be reset. If network connectivity is still missing at reboot, it cannot fence the other... otherwise it’ll join the cluster. Any flaw here? That is +/- the same as with a quorum server... if it cannot reach the quorum server, the node can startup but should not fence the other (which has quorum).

Anyway, I must admit: a quorum server or 3rd node seems safer by design.

Rgds, Dirk

From: mark - pacemaker list [mailto:m+pacemaker at nerdish.us] 
Sent: Friday, November 25, 2011 8:27 PM
To: The Pacemaker cluster resource manager
Subject: Re: [Pacemaker] 2 node cluster questions

Hi Dirk,

On Fri, Nov 25, 2011 at 6:05 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?

Our experience with a two-node SBD-based cluster wasn't good.  After setup, we started on failure scenarios.  The first test was to drop network connectivity for one of the nodes while both could still access storage.  The nodes fenced each other (sort of like a STONITH deathmatch you can read about), killing all services and leaving us waiting for both nodes to boot back up.  Obviously, a complete failure of testing, we didn't even proceed with further checks.  We took a standard PC and built it out as a third node, giving the cluster true quorum, and now it's rock-solid and absolutely correct in every failure scenario we throw at it.  For production use, the very real possibility of two nodes killing each other just wasn't worth the risk to us.

If you go with two nodes and SBD, do a lot of testing.  No matter how much you test though, if they lose visibility to each other on the network but can both still see the storage, you've got a race where the node that *should* be fenced (the one that has its network cables disconnected) can fence the node that is still 100% healthy and actively serving clients.

Maybe there's a way to configure around that, I'd be interested in hearing how if so.

Regards,

Mark

	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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