[ClusterLabs] how to set a dedicated fence delay for a stonith agent ?

[Ken Gaillot]

On 05/10/2017 12:26 PM, Dimitri Maziuk wrote:
> 
>>>>> i remember that digimer often campaigns for a fence delay in a 2-node  cluster.
> ...
>>>>> But  ... a random delay does not seem to
>>>>> be a reliable solution.
> 
>> Some fence agents implement a delay parameter of their own, to set a
>> fixed delay. I believe that's what digimer uses.
> 
> Is it just me or does this sound like catch-22:
> - pacemaker does not work reliably without fencing

Correct -- more specifically, some failure scenarios can't be safely
handled without fencing.

> - fencing in 2-node clusters does not work reliably without fixed delay

Not quite. Fixed delay allows a particular method for avoiding a death
match in a two-node cluster. Pacemaker's built-in random delay
capability is another method.

> - code that ships with pacemaker does not implement fixed delay.

Fence agents are used with pacemaker but not shipped as part of it. They
have their own packages distributed separately. Anyone can write a fence
agent and make it available to the community.

It would be nice if every fence agent supported a delay parameter, but
there's no requirement to do so, and even if there were, it would just
be a guideline -- it's up to the developer.

There's certainly an argument to be made for supporting a fixed delay at
the pacemaker level. There's an idea floating around to do this based on
node health, which could allow a lot of flexibility.