[ClusterLabs] trigger something at ?

[Ken Gaillot]

On Wed, 2024-01-31 at 16:37 +0100, lejeczek via Users wrote:
> 
> On 31/01/2024 16:06, Jehan-Guillaume de Rorthais wrote:
> > On Wed, 31 Jan 2024 16:02:12 +0100
> > lejeczek via Users <users at clusterlabs.org> wrote:
> > 
> > > On 29/01/2024 17:22, Ken Gaillot wrote:
> > > > On Fri, 2024-01-26 at 13:55 +0100, lejeczek via Users wrote:
> > > > > Hi guys.
> > > > > 
> > > > > Is it possible to trigger some... action - I'm thinking
> > > > > specifically
> > > > > at shutdown/start.
> > > > > If not within the cluster then - if you do that - perhaps
> > > > > outside.
> > > > > I would like to create/remove constraints, when cluster
> > > > > starts &
> > > > > stops, respectively.
> > > > > 
> > > > > many thanks, L.
> > > > > 
> > > > You could use node status alerts for that, but it's risky for
> > > > alert
> > > > agents to change the configuration (since that may result in
> > > > more
> > > > alerts and potentially some sort of infinite loop).
> > > > 
> > > > Pacemaker has no concept of a full cluster start/stop, only
> > > > node
> > > > start/stop. You could approximate that by checking whether the
> > > > node
> > > > receiving the alert is the only active node.
> > > > 
> > > > Another possibility would be to write a resource agent that
> > > > does what
> > > > you want and order everything else after it. However it's even
> > > > more
> > > > risky for a resource agent to modify the configuration.
> > > > 
> > > > Finally you could write a systemd unit to do what you want and
> > > > order it
> > > > after pacemaker.
> > > > 
> > > > What's wrong with leaving the constraints permanently
> > > > configured?
> > > yes, that would be for a node start/stop
> > > I struggle with using constraints to move pgsql (PAF) master
> > > onto a given node - seems that co/locating paf's master
> > > results in troubles (replication brakes) at/after node
> > > shutdown/reboot (not always, but way too often)
> > What? What's wrong with colocating PAF's masters exactly? How does
> > it brake any
> > replication? What's these constraints you are dealing with?
> > 
> > Could you share your configuration?
> Constraints beyond/above of what is required by PAF agent 
> itself, say...
> you have multiple pgSQL cluster with PAF - thus multiple 
> (separate, for each pgSQL cluster) masters and you want to 
> spread/balance those across HA cluster
> (or in other words - avoid having more that 1 pgsql master 
> per HA node)
> These below, I've tried, those move the master onto chosen 
> node but.. then the issues I mentioned.
> 
> -> $ pcs constraint location PGSQL-PAF-5438-clone prefers 
> ubusrv1=1002
> or
> -> $ pcs constraint colocation set PGSQL-PAF-5435-clone 
> PGSQL-PAF-5434-clone PGSQL-PAF-5433-clone role=Master 
> require-all=false setoptions score=-1000
> 

Anti-colocation sets tend to be tricky currently -- if the first
resource can't be assigned to a node, none of them can. We have an idea
for a better implementation:

 https://projects.clusterlabs.org/T383

In the meantime, a possible workaround is to use placement-
strategy=balanced and define utilization for the clones only. The
promoted roles will each get a slight additional utilization, and the
cluster should spread them out across nodes whenever possible. I don't
know if that will avoid the replication issues but it may be worth a
try.
-- 
Ken Gaillot <kgaillot at redhat.com>