[ClusterLabs] fence_apc delay?

[Ken Gaillot]

On 09/06/2016 11:44 AM, Dan Swartzendruber wrote:
> On 2016-09-06 10:59, Ken Gaillot wrote:
>> On 09/05/2016 09:38 AM, Marek Grac wrote:
>>> Hi,
>>>
> 
> [snip]
> 
>> FYI, no special configuration is needed for this with recent pacemaker
>> versions. If multiple devices are listed in a topology level, pacemaker
>> will automatically convert reboot requests into all-off-then-all-on.
> 
> Hmmm, thinking about this some more, this just puts me back in the
> current situation (e.g. having an 'extra' delay.)  The issue for me
> would be having two fencing devices, each of which needs a brief delay
> to let its target's PS drain.  If a single PDU fencing agent does this
> (with proposed change):
> 
> power-off
> wait N seconds
> power-on
> 
> that is cool.  Unfortunately, with the all-off-then-all-on pacemaker
> would do, I would get this:
> 
> power-off node A
> wait N seconds
> power-off node B
> wait N seconds
> power-on node A
> power-on node B
> 
> or am I missing something?  If not, seems like it would be nice to have
> some sort of delay at the pacemaker level.  e.g. tell pacemaker to
> convert a reboot of node A into a 'turn off node A, wait N seconds, turn
> on node A'?

You're exactly right. Pacemaker does seem like the appropriate place to
handle this, but it would be a good bit of work. I think the best
workaround for now would be to set the delay only on the B device.

I do see now why power-wait, as a fence agent property, is not ideal for
this purpose: one fence device might be used with multiple nodes, yet
the ideal delay might vary by node (if they have different power supply
models, for example).

On the other hand, setting it as a node attribute isn't right either,
because one node might be fenceable by multiple devices, and the delay
might not be appropriate for all of them.

We'd need to specify the delay per node/device combination -- something
like pcmk_off_delay=node1:3;node2:5 as an (ugly) fence device property.

It would be a significant project. If you think it's important, please
open a feature request at bugs.clusterlabs.org.