[Pacemaker] 2 node cluster questions

[Hellemans Dirk D]

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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