Red Hat Summit13장. Colocating 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]Parameters of a Colocation Constraint
The following table summarizes the properties and options for configuring colocation constraints.
| 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 + |
|
| 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
When this option has a value of
When this option has a value of |
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 constaints 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 ...13.1. Specifying mandatory placement of resources
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.
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