[Pacemaker] Dependency Trees

[Donald Stahl]

> Bad? Choice? What is there for crmsh to choose? This is what is
> produced:
>
>       <rsc_colocation id="s1" score="INFINITY">
>         <resource_set id="s1-0" sequential="false">
>           <resource_ref id="d1"/>
>           <resource_ref id="d2"/>
>         </resource_set>
>       </rsc_colocation>
It does produces the XML above (and which I expected it to produce).
Unfortunately- that XML doesn't do what I want it to do (it actually
seems to have no effect at all).

> Whether that makes sense is another matter.
There does not seem to be any way to say "I want these resources to
all be on the same node" but with absolutely no dependencies.

This:

colocation myset inf: app1 app2

Creates a dependency, implied or otherwise, such that if the second
service can't run, the first won't either. The problem is that even if
they are both running, and I decide to stop the second service for
maintenance reasons, then the first stops as well.

A perfect example is an Oracle Database and listener. They are
independent services that can be stopped and started independently-
but if they are both running- then they need to be running on the same
host. I can find no way to express this in pacemaker. (Well- creating
a group with meta ordered="false" collocated="false" would do it, but
since you can't have groups of groups- you can't create a dependency
tree with multiple such resources).

Now if the Listener and the Database both have the same dependent
services, then yes- they also have to be colocated. But that is an
implied colocation rather than an explicit one. I need to do some more
testing before I can determine how pacemaker behaves under these
conditions as I try to migrate resources.

-Don