[Pacemaker] Proposed new stonith topology syntax

[Digimer]

On 01/18/2012 08:15 AM, Dejan Muhamedagic wrote:
>>> Is there a possibility to express
>>> fencing nodes simultaneously?
>>
>> No.  Its regular boolean shortcut semantics.
> 
> As digimer mentioned, it is one common use case, i.e. for hosts
> with multiple power supplies. So far, we recommended lights-out
> devices for such hardware configurations and if those are
> monitored and more or less reliable such a setup should be fine.
> It would still be good to have a way to express it if some day
> somebody actually implements it. I guess that the schema can be
> easily extended by adding a "simultaneous" attribute to the
> "fencing-rule" element.

If I may restate;

Out of band management devices (iLO, IPMI, w/e) have two fatal flaws
which make them unreliable as sole fence devices; They share their power
with the host and they (generally) have only one network link. If the
node's PSU fails, or if the network link/BMC fails, fencing fails.

A PDU as a backup protects against this, but is not ideal as it can't
confirm a node's power state. This is why I strongly recommend for
people to use ordered fencing; out-of-band management should always be
tried first because if it works, you know for certain the node is dead.
The PDU must be available as a backup, but only be used as such.

This is why I argue so strongly for ordered fencing.

>>> I'd rather use "stonith-resource" than "device", because what is
>>> referenced is a stonith resource (one device may be used in more
>>> than one stonith resource).
>>
>> Can you rephrase that? I don't follow.  Are you talking about a group
>> of fencing devices?
> 
> No, just about naming. The element/attribute name "device"
> doesn't seem right to me, because it references a stonith
> resource. One (physical) device may be used by more than one
> stonith resource. Even though "device" certainly sounds nicer,
> it isn't precise. What I'm worried about is that it may be
> confusing (and we have enough confusion with stonith).
> (Or did I completely misunderstand the meaning of "device"?)
> 
> Thanks,
> 
> Dejan

Red Hat clusters call these "Fence Methods", with each "method"
containing one or more fence "devices". With the IPMI, there is only one
device. With Redundant PSUs across two PDUs, you have two devices in the
"method". All devices in a method must succeed for the fence method to
succeed.

It would, if nothing else, help people migrating to pacemaker from rhcs
if similar names were used.

<fence>
	<method name="ipmi">
		<device name="ipmi_an01" action="reboot" />
	</method>
	<method name="pdu">
		<device name="pdu1" port="1" action="reboot" />
		<device name="pdu2" port="1" action="reboot" />
	</method>
</fence>

-- 
Digimer
E-Mail:              digimer at alteeve.com
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"omg my singularity battery is dead again.
stupid hawking radiation." - epitron