Colocate cluster resources

To specify that the location of one resource depends on the location of another resource, you configure a colocation constraint.

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 colocated 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 [promoted|unpromoted] source_resource with [promoted|unpromoted] target_resource [score] [options]

The following table summarizes the properties and options for configuring colocation constraints:

Table 1. Parameters of a colocation constraint
Parameter Description

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.

influence option

Determines whether the cluster will move both the primary resource (source_resource) and dependent resources (target_resource) to another node when the dependent resource reaches its migration threshold for failure, or whether the cluster will leave the dependent resource offline without causing a service switch.

The influence colocation constraint option can have a value of true or false. The default value for this option is determined by the value of the dependent resource’s critical resource meta option, which has a default value of true.

When this option has a value of true, Pacemaker will attempt to keep both the primary and dependent resource active. If the dependent resource reaches its migration threshold for failures, both resources will move to another node if possible.

When this option has a value of false, Pacemaker will avoid moving the primary resource as a result of the status of the dependent resource. In this case, if the dependent resource reaches its migration threshold for failures, it will stop if the primary resource is active and can remain on its current node.
Displaying colocation constraints

The following command lists all current colocation constraints.

            pcs constraint colocation [config]

You can display all current location, order, and colocation constraints with the following command. To show the internal constraint IDS, specify the --full option.

            pcs constraint [config] [--full]

By default, listing resource constraints does not display expired constraints. To include expired constraints in the listing, use the --all option of the pcs constraint command. This will list expired constraints, noting the constraints and their associated rules as (expired) in the display.

The following command lists the constraints that reference specific resources:

# pcs constraint ref resource ...