[ClusterLabs] Coming in Pacemaker 2.1.0: noncritical resources

[Ken Gaillot]

Hi all,

A recurring request we've seen from Pacemaker users is a feature called
"non-critical resources" in a proprietary product and "independent
subtrees" in the old rgmanager project.

An example is a large database with an occasionally used reporting
tool. The reporting tool is colocated or grouped with the database. If
the reporting tool fails enough times to meet its migration-threshold,
Pacemaker would traditionally move both resources to another node, to
be able to keep them both running.

However, the database may be essential, and take a long time to stop
and start, whereas the reporting tool may not be that important. So,
the user would rather stop the reporting tool in the failure scenario,
rather than cause a database outage to move both.

With the upcoming Pacemaker 2.1.0, this can be controlled with two new
options.

Colocation constraints may take a new "influence" option that
determines whether the dependent resource influences the location of
the main resource, if the main resource is already active. The default
of true preserves the previous behavior. Setting it to false makes the
dependent resource stop rather than move the main resource.

Resources may take a new "critical" meta-attribute that serves as a
default for "influence" in all colocation constraints involving the
resource as the dependent, as well as all groups involving the
resource.

In our above example, either the colocation constraint could be marked
with influence=false, or the reporting tool resource could be give the
meta-attribute critical=false, to achieve the desired effect.

A big list of all changes for 2.1.0 can be found at:

 https://wiki.clusterlabs.org/wiki/Pacemaker_2.1_Changes
-- 
Ken Gaillot <kgaillot at redhat.com>