[DRBD-user] DRBD 9 Primary-Secondary, Pacemaker, and STONITH

[Igor Cicimov]

On Wed, Nov 14, 2018 at 8:31 AM Bryan K. Walton
<bwalton+1539795345 at leepfrog.com> wrote:
>
> I have a two-node DRBD 9 resource configured in Primary-Secondary mode
> with automatic failover configured with Pacemaker.
>
> I know that I need to configure STONITH in Pacemaker and then set DRBD's
> fencing to "resource-and-stonith".
>
> The nodes are Supermicro servers with IPMI.  I'm planning to use IPMI
> for my (first) fencing level.
>
> Where I'm confused is regarding whether I must have a second fencing
> level beyond IPMI?  Or will DRBD's fencing configuration, combined with
> IPMI be good enough?
>
> Looking at:
> http://clusterlabs.org/pacemaker/doc/en-US/Pacemaker/1.1/pdf/Clusters_from_Scratch/Pacemaker-1.1-Clusters_from_Scratch-en-US.pdf
>
> It reads:
>
> "A common mistake people make when choosing a STONITH device
> is to use a remote power switch (such as many on-board IPMI controllers)
> that shares power with the node it controls. If the power fails in such
> a case, the cluster cannot be sure whether the node is really offline,
> or active and suffering from a network fault, so the cluster will stop
> all resources to avoid a possible split-brain situation."
>
> I don't understand this.  If the power fails to a node, then won't the
> node, by definition be down (since there is no power going to the node)?
> So, how then could there be a split brain when one node has no power?

And how is the other node suppose to know that it's peer crashed due
to power failure? From it's stand point the node disappeared and it
has to attempt to STONITH. Now, if the STONITH device on the other
side gets it's power via the same supply as the server then STONITH is
not possible because the stonith device itself will be powered down
too. You see the problem now?

>
>
> Is the above quote stating that if Pacemaker can't confirm that one
> node has been STONITHed, that it won't allow the remaining node to work,
> either?
>
> Thanks!
> Bryan Walton
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