8.6. Using Rules to Determine Resource Location

⁠8.6. Using Rules to Determine Resource Location

A location constraint may contain one or more top-level rules. The cluster will act as if there is a separate location constraint for each rule that evaluates as true.

Consider the following simple location constraint:

⁠

Example 8.12. Prevent resource "webserver" from running on node3

<rsc_location id="ban-apache-on-node3" rsc="webserver"
              score="-INFINITY" node="node3"/>

The constraint can be more verbosely written using a rule:

⁠

Example 8.13. Prevent resource "webserver" from running on node3 using rule

<rsc_location id="ban-apache-on-node3" rsc="webserver">
    <rule id="ban-apache-rule" score="-INFINITY">
      <expression id="ban-apache-expr" attribute="#uname"
        operation="eq" value="node3"/>
    </rule>
</rsc_location>

The advantage of using the expanded form is that one could add more expressions (for example, limiting the constraint to certain days of the week), or activate the constraint by some node attribute other than node name.

⁠8.6.1. Location Rules Based on Other Node Properties

The expanded form allows us to match on node properties other than its name. If we rated each machine’s CPU power such that the cluster had the following nodes section:

⁠

Example 8.14. A sample nodes section for use with score-attribute

<nodes>
   <node id="uuid1" uname="c001n01" type="normal">
      <instance_attributes id="uuid1-custom_attrs">
        <nvpair id="uuid1-cpu_mips" name="cpu_mips" value="1234"/>
      </instance_attributes>
   </node>
   <node id="uuid2" uname="c001n02" type="normal">
      <instance_attributes id="uuid2-custom_attrs">
        <nvpair id="uuid2-cpu_mips" name="cpu_mips" value="5678"/>
      </instance_attributes>
   </node>
</nodes>

then we could prevent resources from running on underpowered machines with this rule:

<rule id="need-more-power-rule" score="-INFINITY">
   <expression id="need-more-power-expr" attribute="cpu_mips"
               operation="lt" value="3000"/>
</rule>