[ClusterLabs] questions about startup fencing

Kristoffer Grönlund kgronlund at suse.com
Wed Nov 29 13:11:29 EST 2017

Adam Spiers <aspiers at suse.com> writes:

> Kristoffer Gronlund <kgronlund at suse.com> wrote:
>>Adam Spiers <aspiers at suse.com> writes:
>>> - The whole cluster is shut down cleanly.
>>> - The whole cluster is then started up again.  (Side question: what
>>>   happens if the last node to shut down is not the first to start up?
>>>   How will the cluster ensure it has the most recent version of the
>>>   CIB?  Without that, how would it know whether the last man standing
>>>   was shut down cleanly or not?)
>>This is my opinion, I don't really know what the "official" pacemaker
>>stance is: There is no such thing as shutting down a cluster cleanly. A
>>cluster is a process stretching over multiple nodes - if they all shut
>>down, the process is gone. When you start up again, you effectively have
>>a completely new cluster.
> Sorry, I don't follow you at all here.  When you start the cluster up
> again, the cluster config from before the shutdown is still there.
> That's very far from being a completely new cluster :-)

You have a new cluster with (possibly fragmented) memories of a previous
life ;)

> Yes, exactly.  If the first node to start up was not the last man
> standing, the CIB history is effectively being forked.  So how is this
> issue avoided?
>>The only way to bring up a cluster from being completely stopped is to
>>treat it as creating a completely new cluster. The first node to start
>>"creates" the cluster and later nodes join that cluster.
> That's ignoring the cluster config, which persists even when the
> cluster's down.

There could be a command in pacemaker which resets a set of nodes to a
common known state, basically to pick the CIB from one of the nodes as
the survivor and copy that to all of them. But in the end, that's just
the same thing as just picking one node as the first node, and telling
the others to join that one and to discard their configurations. So,
treating it as a new cluster.

> But to be clear, you picked a small side question from my original
> post and answered that.  The main questions I had were about startup
> fencing :-)

I did! :)

// Kristoffer Grönlund
// kgronlund at suse.com

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