[Pacemaker] Order Constraint for: stop rscA only when rscB is running ?

[Andreas Kurz]

On Thursday 09 October 2008 16:43:05 Andrew Beekhof wrote:
> On Oct 9, 2008, at 3:55 PM, Andreas Kurz wrote:
> > On Thursday 09 October 2008 13:58:26 Andrew Beekhof wrote:
> >> On Oct 9, 2008, at 12:40 PM, Andreas Kurz wrote:
> >>> On Monday 06 October 2008 15:36:31 Andrew Beekhof wrote:
> >>>> On Thu, Oct 2, 2008 at 10:23, Andreas Kurz
> >>>>
> >>>> <andreas.kurz at linbit.com> wrote:
> >>>>> Hi,
> >>>>>
> >>>>> I have a Heartbeat 2.1.3 two-node cluster and I wonder if there is
> >>>>> a way
> >>>>> to create constraints that work like that:
> >>>>>
> >>>>> * Two resources (rscA and rscB) of class "heartbeat" that should
> >>>>> run
> >>>>> together on one node .... simple advisory colocation constraint
> >>>>>
> >>>>> <rsc_colocation id="rscA-colocated-with-rscB"
> >>>>>      from="rscA" to="rscB"
> >>>>>      score="1" symmetrical="true"/>
> >>>>>
> >>>>> * When doing maintenance on one node I put the node into standby
> >>>>> mode and
> >>>>> want rscA only stop and migrate away if rscB is already up and
> >>>>> running on
> >>>>> the other node and vice versa because  there must always run at
> >>>>> least one
> >>>>> of these resources in the cluster.
> >>>>>
> >>>>> I added two order_constraints and tested it with ptest on an
> >>>>> adapted live
> >>>>> cib but they seem to produce a loop:
> >>>>
> >>>> This is logical if you draw it out (and remember that Stop X always
> >>>> precedes Start X)
> >>>
> >>> Yes, I expected a loop and I had a slide hope there is some
> >>> cool "prevent-loops" logic available ;-)
> >>
> >> Possible in theory - but there is no way to know where the "right"
> >> place to break the chain is.
> >
> > Maybe using the priority of the resources to break the chain?
>
> Still wont help you, since every link in the chain is "required".

OK,  I see ....

>
> >>>> Any reason not to just use the following?
> >>>>
> >>>> <rsc_order id="always-start-A-after-B"
> >>>>      from="service_rscB" action="start"
> >>>>      type="before"
> >>>>      to="service_rscA" to_action="start"
> >>>>      symmetrical="true" score="0"/>
> >>>
> >>> The reason is that first rscA then rscB ist stopped ... so they are
> >>> both down
> >>> before they are restarted in this order on the other node.
> >>>
> >>> The idea is ... in case of a manual switchover of all services from
> >>> nodeA to
> >>> nodeB during regular failover tests, rscA should be stopped on nodeA
> >>> _after_
> >>> rscB is already _started_ on nodeB.
> >>
> >> But in normal operation rscB should start after rscA?
> >
> > This is not necessary, the should run together on one node in normal
> > operation, for a short period of time they can run on different
> > nodes ... the
> > only realy demand is: keep at least one of these resources running
> >
> >>>>> <rsc_order id="stop-rscA-only-after-rscB-start"
> >>>>>      from="service_rscA" action="stop"
> >>>>>      type="after"
> >>>>>      to="service_rscB" to_action="start"
> >>>>>      symmetrical="false" score="0"/>
>
> Then just this constraint should work should it not?
>
> If A and B are both restarting, then A will always wait for B to be
> active.
> If only A is restarting, then B will still be active.
> If only B is restarting, then A will still be active.
>
> If A and B are stopping, the constraint is ignored.
> If A is stopping, and B remains (in)active, the constraint is ignored.
> If B is stopping, and A remains (in)active, the constraint is ignored.
>
> Did I miss something?

Tested it, worked as expected ;-) ... Thanks Andrew!

Regards,
Andreas

-- 
: Andreas Kurz					 
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