[Pacemaker] Split Site 2-way clusters

[Miki Shapiro]

Separate to my earlier post re CRM DC election in a 2-way cluster, I'm chasing up the (separate) issue of making the cluster a CROSS-SITE one.

As stated in yay other thread, I'm running a 2-way quorum-agnostic cluster on a SLES11, openais, pacemaker, drbd (... clvm, ocfs2, ctdb, nfs, etc) on HP Blades.

A few old threads (with a rather elaborate answer from Lars) indicate that as of March 2009 split-site wasn't yet thoroughly supported as WAN connectivity issues were not thoroughly addressed, and that as of then quorumd was not yet sufficiently robust/tested/PROD-ready.

What we decided we want to do is rely on an extremely simple (and hopefully by inference predictable and reliable) arbitrator - a THIRD linux server that lives at a SEPARATE THIRD site altogether with no special HA-related daemons running on it.

I'll build a STONITH ocf script, configure it as a cloned STONITH resource running on both nodes, and it will do roughly this when pinging the other node (via either one or two redundant links) will fail:

ssh arbitrator mkdir /tmp/$clustername && shoot-other-node || hard-suicide-NOW

Thus, when split, the nodes race to the arbitrator.
First to run the mkdir command on the arbitrator (and get rc=0) wins, gets the long straw and lives.  Loser gets shot (either by its peer if WAN allows peer to communicate with soon-to-be-dead node's iLO or by said node sealing its own fate).

Primary failure modes not accounted for by a run-of-the-mill non-split-site cluster are thus:


1.       One node cut off - cutoff node will fail the race and suicide. Good node will succeed and proceed to provide service.

2.       Nodes cut off from each other but can both access the arbitrator - slower node will suicide. Faster node will succeed and proceed to provide service.

3.       Both nodes are cut off, or the comms issue affects both node1<->node2 comms AND all ->arbitrator comms (double failure).  - both nodes suicide (and potentially leave me with two inconsistent and potentially corrupt filesystems). Can't see an easy way around this one (can anyone?)



Looks to me like this can easily be implemented without any fancy quorum servers (on top of the required little ocf script and the existence of the arbitrator)

Does anyone have thoughts on this? Am I ignoring any major issues, or reinventing the wheel, or should this this potentially work as I think it will?

Thanks! :)

And a little addendum which just occurred to me re transient WAN network issues:



1.       Transient big (>2min) network issues will land me with a cluster that needs a human to turn on one node on every time they happen. Bad.



My proposed solution: classify a peer-failure as a WAN-problem by pinging peer node's core router when peer node appears dead, if router dead too touch a WAN-problem-flagfile, and so long as the flag-file sits there the survivor pings (done via ocf ping resource) other-side-router until it comes online, then shooting a "check-power-status && O-GOD-IT-STILL-LIVES-KILL-IT-NOW || power-it-on" command to the peer's iLO (and promptly delete the flag).



Implementation cost: a wee bit of scripting and a wee bit of pacemaker configuration.



2.       Transient small network issues will require stretching pacemaker's default timeouts sufficiently to avoid (or end up in the item 1 bucket above)

Am very keen to know what the gurus think :) :)

Miki Shapiro
Linux Systems Engineer
Infrastructure Services & Operations

[cid:image001.png at 01CA9473.B8C182C0]
745 Springvale Road
Mulgrave 3170 Australia
Email miki.shapiro at coles.com.au<mailto:miki.shapiro at coles.com.au>
Phone: 61 3 854 10520
Fax:     61 3 854 10558



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