[ClusterLabs] Coming in Pacemaker 2.0.4: fencing delay based on what resources are where

[Andrei Borzenkov]

21.03.2020 20:07, Ken Gaillot пишет:
> Hi all,
> 
> I am happy to announce a feature that was discussed on this list a
> while back. It will be in Pacemaker 2.0.4 (the first release candidate
> is expected in about three weeks).
> 
> A longstanding concern in two-node clusters is that in a split brain,
> one side must get a fencing delay to avoid simultaneous fencing of both
> nodes, but there is no perfect way to determine which node gets the
> delay.
> 
> The most common approach is to configure a static delay on one node.
> This is particularly useful in an active/passive setup where one
> particular node is normally assigned the active role.
> 
> Another approach is to use the relatively new fence_heuristics_ping
> agent in a topology with your real fencing agent. A node that can ping
> a configured IP will be more likely to survive.
> 
> In addition, we now have a new cluster-wide property, priority-fencing-
> delay, that bases the delay on what resources were known to be active
> where just before the split. If you set the new property, and configure
> priorities for your resources, the node with the highest combined
> priority of all resources running on it will be more likely to survive.
> 
> As an example, if you set a default priority of 1 for all resources,
> and set priority-fencing-delay to 15s, then the node running the most
> resources will be more likely to survive because the other node will
> wait 15 seconds before initiating fencing. If a particular resource is
> more important than the rest, you can give it a higher priority.
> 

That sounds good except one consideration. "priority" also affects
resource placement, and changing it may have rather unexpected results,
especially in cases when scores are carefully selected to achieve
resource distribution.

> The master role of promotable clones will get an extra 1 point, if a
> priority has been configured for that clone.
> 
> If both nodes have equal priority, or fencing is needed for some reason
> other than node loss (e.g. on-fail=fencing for some monitor), then the
> usual delay properties apply (pcmk_delay_base, etc.).
> 
> I'd like to recognize the primary authors of the 2.0.4 features
> announced so far:
> - shutdown locks: myself
> - switch to clock_gettime() for monotonic clock: Jan Pokorný
> - crm_mon --include/--exclude: Chris Lumens
> - priority-fencing-delay: Gao,Yan
>