[Linux-cluster] Qdisk question

[brem belguebli]

Hi Lon and Thanks for this reply.

In fact, thinking about it, my test wasn't very much representative of what
I was expecting to do.

I blocked the qdisk communications to only one node which, after reading
your reply, kind of confirmed me that I did the wong test. I'm going to re
run it by blocking all the nodes to the qdisk.

I'll also try your ping tie-breaker.

Brem


2009/8/13, Lon Hohberger <lhh at redhat.com>:
>
> On Thu, 2009-08-13 at 00:45 +0200, brem belguebli wrote:
>
> > My understanding of qdisk is that it is used as a tie-breaker, but it
> > looks like it is more a heatbeat vector than a simple tie-breaker.
>
> Right, it's a secondary membership algorithm.
>
>
> > Until here, no real problem indeed, if the site gets apart from the
> > other prod site and also from the third site (hosting the iscsi target
> > qdisk) the 2 nodes from the failing site get evicted from the cluster.
> >
> >
> > But, what if my third site gets isolated while the 2 prod ones are
> > fine ?
>
> Qdisk votes will not be presented to CMAN any more, but the two sites
> should remain online if they still have a "majority" of votes.
>
>
> > The real  question is what happens in case all the nodes loose access
> > to the qdisk while they're still able to see each others ?
>
> Qdisk is just a vote like other voting mechanisms.  If all nodes lose
> access at the same time, it should behave like a node death.  However,
> the default action if _one_ node loses access is to kill that node (even
> if CMAN still sees it).
>
>
> > The 4 nodes have each 1 vote and the qdisk 1 vote. The expected quorum
> > is 3.
>
>
> > If I loose the qdisk, the number of votes falls to 4, the cluster is
> > quorate (4>3) but it looks like everything goes bad, each node
> > deactivate itself as it can't write its alive status (--> heartbeat
> > vector) to the qdisk even if the network heartbeating is working
> > fine.
>
> What happens specifically?  Most of the actions qdiskd performs are
> configurable.  For example, if the nodes are rebooting, you can turn
> that behavior off.
>
>
>
> I wrote a simple 'ping' tiebreaker based the behaviors in RHEL3.  It
> functions in many ways in the same manner as qdiskd with respect to vote
> advertisement to CMAN, but without needing a disk - maybe you would find
> it useful?
>
> http://people.redhat.com/lhh/qnet.tar.gz
>
> -- Lon
>
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