6.3. Colocation of Resources

A colocation constraint determines that the location of one resource depends on the location of another resource.
There is an important side effect of creating a colocation constraint between two resources: it affects the order in which resources are assigned to a node. This is because you cannot place resource A relative to resource B unless you know where resource B is. So when you are creating colocation constraints, it is important to consider whether you should colocate resource A with resource B or resource B with resource A.
Another thing to keep in mind when creating colocation constraints is that, assuming resource A is collocated with resource B, the cluster will also take into account resource A's preferences when deciding which node to choose for resource B.
The following command creates a colocation constraint.
pcs constraint colocation add [master|slave] source_resource with [master|slave] target_resource [score] [options]
For information on master and slave resources, see Section 8.2, “Multi-State Resources: Resources That Have Multiple Modes”.
Table 6.3, “Properties of a Colocation Constraint”. summarizes the properties and options for configuring colocation constraints.

Table 6.3. Properties of a Colocation Constraint

FieldDescription
source_resource
The colocation source. If the constraint cannot be satisfied, the cluster may decide not to allow the resource to run at all.
target_resource
The colocation target. The cluster will decide where to put this resource first and then decide where to put the source resource.
score
Positive values indicate the resource should run on the same node. Negative values indicate the resources should not run on the same node. A value of + INFINITY, the default value, indicates that the source_resource must run on the same node as the target_resource. A value of - INFINITY indicates that the source_resource must not run on the same node as the target_resource.

6.3.1. Mandatory Placement

Mandatory placement occurs any time the constraint's score is +INFINITY or -INFINITY. In such cases, if the constraint cannot be satisfied, then the source_resource is not permitted to run. For score=INFINITY, this includes cases where the target_resource is not active.
If you need myresource1 to always run on the same machine as myresource2, you would add the following constraint:
# pcs constraint colocation add myresource1 with myresource2 score=INFINITY
Because INFINITY was used, if myresource2 cannot run on any of the cluster nodes (for whatever reason) then myresource1 will not be allowed to run.
Alternatively, you may want to configure the opposite, a cluster in which myresource1 cannot run on the same machine as myresource2. In this case use score=-INFINITY
# pcs constraint colocation add myresource1 with myresource2 score=-INFINITY
Again, by specifying -INFINITY, the constraint is binding. So if the only place left to run is where myresource2 already is, then myresource1 may not run anywhere.