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Chapter 24. Configuring a node placement strategy

Pacemaker decides where to place a resource according to the resource allocation scores on every node. The resource will be allocated to the node where the resource has the highest score. This allocation score is derived from a combination of factors, including resource constraints, resource-stickiness settings, prior failure history of a resource on each node, and utilization of each node.

If the resource allocation scores on all the nodes are equal, by the default placement strategy Pacemaker will choose a node with the least number of allocated resources for balancing the load. If the number of resources on each node is equal, the first eligible node listed in the CIB will be chosen to run the resource.

Often, however, different resources use significantly different proportions of a node’s capacities (such as memory or I/O). You cannot always balance the load ideally by taking into account only the number of resources allocated to a node. In addition, if resources are placed such that their combined requirements exceed the provided capacity, they may fail to start completely or they may run with degraded performance. To take these factors into account, Pacemaker allows you to configure the following components:

  • the capacity a particular node provides
  • the capacity a particular resource requires
  • an overall strategy for placement of resources

To configure the capacity that a node provides or a resource requires, you can use utilization attributes for nodes and resources. You do this by setting a utilization variable for a resource and assigning a value to that variable to indicate what the resource requires, and then setting that same utilization variable for a node and assigning a value to that variable to indicate what that node provides.

You can name utilization attributes according to your preferences and define as many name and value pairs as your configuration needs. The values of utilization attributes must be integers.

Configuring node and resource capacity

The following example procedure configures utilization attributes for two nodes and then specifies the same utilization attributes that three different resources require. A node is considered eligible for a resource if it has sufficient free capacity to satisfy the resource’s requirements, as defined by the utilization attributes.

Displaying node utilization as pcs commands

You can export node utilization as a series of pcs commands by using the --output-format=cmd option. This is useful for scripting, automation, or replicating the same configuration on a different system.

Note

You can display the configured node utilization in one of three formats:

  • text: Displays the output in plain text. This is the default format.
  • json: Displays the output in a machine-readable JSON format, which is useful for scripting and automation.
  • cmd: Displays the output as a series of pcs commands, which you can use to recreate the same node utilization on a different system.

  • To display the configured node utilization as a series of pcs commands: 

    # pcs node utilization --output-format=cmd

    Example output

    pcs node utilization node1 cpu=2
pcs node utilization node2 cpu=4

Configuring placement strategy

After you have configured the capacities your nodes provide and the capacities your resources require, you need to set the placement-strategy cluster property, otherwise the capacity configurations have no effect.

Four values are available for the placement-strategy cluster property:

  • default - Utilization values are not taken into account at all. Resources are allocated according to allocation scores. If scores are equal, resources are evenly distributed across nodes.
  • utilization - Utilization values are taken into account only when deciding whether a node is considered eligible (that is, whether it has sufficient free capacity to satisfy the resource’s requirements). Load-balancing is still done based on the number of resources allocated to a node.
  • balanced - Utilization values are taken into account when deciding whether a node is eligible to serve a resource and when load-balancing, so an attempt is made to spread the resources in a way that optimizes resource performance.
  • minimal - Utilization values are taken into account only when deciding whether a node is eligible to serve a resource. For load-balancing, an attempt is made to concentrate the resources on as few nodes as possible, thereby enabling possible power savings on the remaining nodes.

The following example command sets the value of placement-strategy to balanced. After running this command, Pacemaker will ensure the load from your resources will be distributed evenly throughout the cluster, without the need for complicated sets of colocation constraints. 

# pcs property set placement-strategy=balanced

Procedure

  1. Configure utilization attributes for node1. This command configures a node attribute of CPU capacity, setting this attribute as the variable cpu and defining node1 as providing a CPU capacity of two. This command also defines a utilization attribute of RAM capacity, setting this attribute as the variable memory and defining node1 as providing a RAM capacity of 2048. 

    # pcs node utilization node1 cpu=2 memory=2048

  2. Configure utilization attributes for node2. This command configures a node attribute of CPU capacity, setting this attribute as the variable cpu and defining node2 as providing a CPU capacity of four. This command also defines a utilization attribute of RAM capacity, setting this attribute as the variable memory and defining node2 as providing a RAM capacity of 2048. 

    # pcs node utilization node2 cpu=4 memory=2048

  3. Specify the utilization attributes that resource dummy-small requires. In this example, dummy-small requires a CPU capacity of 1 and a RAM capacity of 1024. 

    # pcs resource utilization dummy-small cpu=1 memory=1024

  4. Specify the utilization attributes that resource dummy-medium requires. In this example, dummy-medium requires a CPU capacity of 2 and a RAM capacity of 2048. 

    # pcs resource utilization dummy-medium cpu=2 memory=2048

  5. Specify the utilization attributes that resource dummy-large requires. In this example, dummy-requires requires a CPU capacity of 1 and a RAM capacity of 3072. 

    # pcs resource utilization dummy-large cpu=3 memory=3072

24.2. Pacemaker resource allocation
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Pacemaker allocates resources according to node preference, node capacity, and resource allocation preference.

24.2.1. Node preference
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Pacemaker determines which node is preferred when allocating resources according to the following strategy.

  • The node with the highest node weight gets consumed first. Node weight is a score maintained by the cluster to represent node health.

  • If multiple nodes have the same node weight:

    • If the placement-strategy cluster property is default or utilization:

      • The node that has the least number of allocated resources gets consumed first.
      • If the numbers of allocated resources are equal, the first eligible node listed in the CIB gets consumed first.

    • If the placement-strategy cluster property is balanced:

      • The node that has the most free capacity gets consumed first.
      • If the free capacities of the nodes are equal, the node that has the least number of allocated resources gets consumed first.
      • If the free capacities of the nodes are equal and the number of allocated resources is equal, the first eligible node listed in the CIB gets consumed first.
    • If the placement-strategy cluster property is minimal, the first eligible node listed in the CIB gets consumed first.

24.2.2. Node capacity
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Pacemaker determines which node has the most free capacity according to the following strategy.

  • If only one type of utilization attribute has been defined, free capacity is a simple numeric comparison.

  • If multiple types of utilization attributes have been defined, then the node that is numerically highest in the most attribute types has the most free capacity. For example:

    • If NodeA has more free CPUs, and NodeB has more free memory, then their free capacities are equal.
    • If NodeA has more free CPUs, while NodeB has more free memory and storage, then NodeB has more free capacity.

24.2.3. Resource allocation preference
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Pacemaker determines which resource is allocated first according to the following strategy.

  • The resource that has the highest priority gets allocated first. You can set a resource’s priority when you create the resource.
  • If the priorities of the resources are equal, the resource that has the highest score on the node where it is running gets allocated first, to prevent resource shuffling.
  • If the resource scores on the nodes where the resources are running are equal or the resources are not running, the resource that has the highest score on the preferred node gets allocated first. If the resource scores on the preferred node are equal in this case, the first runnable resource listed in the CIB gets allocated first.

24.3. Resource placement strategy guidelines
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Optimize Pacemaker’s resource placement strategy by ensuring sufficient physical capacity, configuring buffers for overcommitment, and defining clear resource priorities.

  • Make sure that you have sufficient physical capacity.

    If the physical capacity of your nodes is being used to near maximum under normal conditions, then problems could occur during failover. Even without the utilization feature, you may start to experience timeouts and secondary failures.

  • Build some buffer into the capabilities you configure for the nodes.

    Advertise slightly more node resources than you physically have, on the assumption the that a Pacemaker resource will not use 100% of the configured amount of CPU, memory, and so forth all the time. This practice is sometimes called overcommit.

  • Specify resource priorities.

    If the cluster is going to sacrifice services, it should be the ones you care about least. Ensure that resource priorities are properly set so that your most important resources are scheduled first.

24.4. The NodeUtilization resource agent
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The NodeUtilization resoure agent can detect the system parameters of available CPU, host memory availability, and hypervisor memory availability and add these parameters into the CIB. You can run the agent as a clone resource to have it automatically populate these parameters on each node.

For information about the NodeUtilization resource agent and the resource options for this agent, run the pcs resource describe NodeUtilization command.

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