11.3. Configuring a location constraint strategy

When using location constraints, you can configure a general strategy for specifying which nodes a resource can run on:

Whether you should choose to configure your cluster as an opt-in or opt-out cluster depends on both your personal preference and the make-up of your cluster. If most of your resources can run on most of the nodes, then an opt-out arrangement is likely to result in a simpler configuration. On the other hand, if most resources can only run on a small subset of nodes an opt-in configuration might be simpler.

Configuring an "Opt-In" cluster

To create an opt-in cluster, set the symmetric-cluster cluster property to false to prevent resources from running anywhere by default.

# pcs property set symmetric-cluster=false

Enable nodes for individual resources. The following commands configure location constraints so that the resource Webserver prefers node example-1, the resource Database prefers node example-2, and both resources can fail over to node example-3 if their preferred node fails. When configuring location constraints for an opt-in cluster, setting a score of zero allows a resource to run on a node without indicating any preference to prefer or avoid the node.

# pcs constraint location Webserver prefers example-1=200
# pcs constraint location Webserver prefers example-3=0
# pcs constraint location Database prefers example-2=200
# pcs constraint location Database prefers example-3=0

Configuring an "Opt-Out" cluster

To create an opt-out cluster, set the symmetric-cluster cluster property to true to allow resources to run everywhere by default. This is the default configuration if symmetric-cluster is not set explicitly.

# pcs property set symmetric-cluster=true

The following commands will then yield a configuration that is equivalent to the example in "Configuring an "Opt-In" cluster". Both resources can fail over to node example-3 if their preferred node fails, since every node has an implicit score of 0.

# pcs constraint location Webserver prefers example-1=200
# pcs constraint location Webserver avoids example-2=INFINITY
# pcs constraint location Database avoids example-1=INFINITY
# pcs constraint location Database prefers example-2=200

Note that it is not necessary to specify a score of INFINITY in these commands, since that is the default value for the score.