[Linux-cluster] new cluster acting odd

Digimer lists at alteeve.ca
Mon Dec 1 17:02:43 UTC 2014

On 01/12/14 11:34 AM, Dan Riley wrote:
> Ha, I was unaware this was part of the folklore.  We have a couple of 9-node clusters, it did take some tuning to get them stable, and we are thinking about splitting one of them.  For our clusters, we found uniform configuration helped a lot, so a mix of physical hosts and VMs in the same (largish) cluster would make me a little nervous, don't know about anyone else's feelings.

Personally, I only build 2-node clusters. When I need more resource, I 
drop-in another pair. This allows all my clusters, going back to 2008/9, 
to have nearly identical configurations. In HA, I would argue, nothing 
is more useful than consistency and simplicity.

That said, I'd not fault anyone for going to 4 or 5 node. Beyond that 
though, I would argue that the cluster should be broken up. In HA, 
uptime should always trump resource utilization efficiency.

Mixing real and virtual machines strikes me as an avoidable complexity, too.

>> Something fence related is not working.
> We used to see something like this, and it usually traced back to "shouldn't be possible" inconsistencies in the fence group membership.  Once the fence group gets blocked by a mismatched membership list, everything above it breaks.  Sometimes a "fence_tool ls -n" on all the cluster members will reveal an inconsistency ("fence_tool dump" also, but that's hard to interpret without digging into the group membership protocols).  If you can find an inconsistency, manually fencing the nodes in the minority might repair it.

In all my years, I've never seen this happen in production. If you can 
create a reproducer, I would *love* to see/examine it!

> At the time, I did quite a lot of staring at fence_tool dumps, but never figured out how the fence group was getting into "shouldn't be possible" inconsistencies.  This was also all late RHEL5 and early RHEL6, so may not be applicable anymore.

HA in 6.2+ seems to be quite a bit more stable (I think for more reasons 
than just the HA stuff). For this reason, I am staying on RHEL 6 until 
at least 7.2+ is out. :)

>> My recommendation would be to schedule a maintenance window and then stop everything except cman (no rgmanager, no gfs2, etc). Then methodically test crashing all nodes (I like 'echo c > /proc/sysrq-trigger) and verify they are fenced and then recover properly. It's worth disabling cman and rgmanager from starting at boot (period, but particularly for this test).
>> If you can reliably (and repeatedly) crash -> fence -> rejoin, then I'd start loading back services and re-trying. If the problem reappears only under load, then that's an indication of the problem, too.
> I'd agree--start at the bottom of the stack and work your way up.
> -dan

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