3. Cluster Nodes¶

3.1. Defining a Cluster Node¶

Each cluster node will have an entry in the nodes section containing at least an ID and a name. A cluster node’s ID is defined by the cluster layer (Corosync).

Example Corosync cluster node entry

<node id="101" uname="pcmk-1"/>

In normal circumstances, the admin should let the cluster populate this information automatically from the cluster layer.

3.1.1. Where Pacemaker Gets the Node Name¶

The name that Pacemaker uses for a node in the configuration does not have to be the same as its local hostname. Pacemaker uses the following for a Corosync node’s name, in order of most preferred first:

The value of name in the nodelist section of corosync.conf
The value of ring0_addr in the nodelist section of corosync.conf
The local hostname (value of uname -n)

If the cluster is running, the crm_node -n command will display the local node’s name as used by the cluster.

If a Corosync nodelist is used, crm_node --name-for-id with a Corosync node ID will display the name used by the node with the given Corosync nodeid, for example:

crm_node --name-for-id 2

3.2. Node Attributes¶

Pacemaker allows node-specific values to be specified using node attributes. A node attribute has a name, and may have a distinct value for each node.

Node attributes come in two types, permanent and transient. Permanent node attributes are kept within the node entry, and keep their values even if the cluster restarts on a node. Transient node attributes are kept in the CIB’s status section, and go away when the cluster stops on the node.

While certain node attributes have specific meanings to the cluster, they are mainly intended to allow administrators and resource agents to track any information desired.

For example, an administrator might choose to define node attributes for how much RAM and disk space each node has, which OS each uses, or which server room rack each node is in.

Users can configure Rules that use node attributes to affect where resources are placed.

3.2.1. Setting and querying node attributes¶

Node attributes can be set and queried using the crm_attribute and attrd_updater commands, so that the user does not have to deal with XML configuration directly.

Here is an example command to set a permanent node attribute, and the XML configuration that would be generated:

Result of using crm_attribute to specify which kernel pcmk-1 is running

# crm_attribute --type nodes --node pcmk-1 --name kernel --update $(uname -r)

<node id="1" uname="pcmk-1">
   <instance_attributes id="nodes-1-attributes">
     <nvpair id="nodes-1-kernel" name="kernel" value="3.10.0-862.14.4.el7.x86_64"/>
   </instance_attributes>
</node>

To read back the value that was just set:

# crm_attribute --type nodes --node pcmk-1 --name kernel --query
scope=nodes  name=kernel value=3.10.0-862.14.4.el7.x86_64

The --type nodes indicates that this is a permanent node attribute; --type status would indicate a transient node attribute.

3.2.2. Special node attributes¶

Certain node attributes have special meaning to the cluster.

Node attribute names beginning with # are considered reserved for these special attributes. Some special attributes do not start with #, for historical reasons.

Certain special attributes are set automatically by the cluster, should never be modified directly, and can be used only within Rules; these are listed under built-in node attributes.

For true/false values, the cluster considers a value of “1”, “y”, “yes”, “on”, or “true” (case-insensitively) to be true, “0”, “n”, “no”, “off”, “false”, or unset to be false, and anything else to be an error.

**Node attributes with special significance**¶
Name	Description
fail-count-*	Attributes whose names start with `fail-count-` are managed by the cluster to track how many times particular resource operations have failed on this node. These should be queried and cleared via the `crm_failcount` or `crm_resource --cleanup` commands rather than directly.
last-failure-*	Attributes whose names start with `last-failure-` are managed by the cluster to track when particular resource operations have most recently failed on this node. These should be cleared via the `crm_failcount` or `crm_resource --cleanup` commands rather than directly.
maintenance	Similar to the `maintenance-mode` cluster option, but for a single node. If true, resources will not be started or stopped on the node, resources and individual clone instances running on the node will become unmanaged, and any recurring operations for those will be cancelled. Warning: Restarting pacemaker on a node that is in single-node maintenance mode will likely lead to undesirable effects. If `maintenance` is set as a transient attribute, it will be erased when Pacemaker is stopped, which will immediately take the node out of maintenance mode and likely get it fenced. Even if permanent, if Pacemaker is restarted, any resources active on the node will have their local history erased when the node rejoins, so the cluster will no longer consider them running on the node and thus will consider them managed again, leading them to be started elsewhere. This behavior might be improved in a future release.
probe_complete	This is managed by the cluster to detect when nodes need to be reprobed, and should never be used directly.
resource-discovery-enabled	If the node is a remote node, fencing is enabled, and this attribute is explicitly set to false (unset means true in this case), resource discovery (probes) will not be done on this node. This is highly discouraged; the `resource-discovery` location constraint property is preferred for this purpose.
shutdown	This is managed by the cluster to orchestrate the shutdown of a node, and should never be used directly.
site-name	If set, this will be used as the value of the `#site-name` node attribute used in rules. (If not set, the value of the `cluster-name` cluster option will be used as `#site-name` instead.)
standby	If true, the node is in standby mode. This is typically set and queried via the `crm_standby` command rather than directly.
terminate	If the value is true or begins with any nonzero number, the node will be fenced. This is typically set by tools rather than directly.
#digests-*	Attributes whose names start with `#digests-` are managed by the cluster to detect when Unfencing needs to be redone, and should never be used directly.
#node-unfenced	When the node was last unfenced (as seconds since the epoch). This is managed by the cluster and should never be used directly.

3.3. Tracking Node Health¶

A node may be functioning adequately as far as cluster membership is concerned, and yet be “unhealthy” in some respect that makes it an undesirable location for resources. For example, a disk drive may be reporting SMART errors, or the CPU may be highly loaded.

Pacemaker offers a way to automatically move resources off unhealthy nodes.

3.3.1. Node Health Attributes¶

Pacemaker will treat any node attribute whose name starts with #health as an indicator of node health. Node health attributes may have one of the following values:

**Allowed Values for Node Health Attributes**¶
Value	Intended significance
`red`	This indicator is unhealthy
`yellow`	This indicator is becoming unhealthy
`green`	This indicator is healthy
integer	A numeric score to apply to all resources on this node (0 or positive is healthy, negative is unhealthy)

3.3.2. Node Health Strategy¶

Pacemaker assigns a node health score to each node, as the sum of the values of all its node health attributes. This score will be used as a location constraint applied to this node for all resources.

The node-health-strategy cluster option controls how Pacemaker responds to changes in node health attributes, and how it translates red, yellow, and green to scores.

Allowed values are:

**Node Health Strategies**¶
Value	Effect
none	Do not track node health attributes at all.
migrate-on-red	Assign the value of `-INFINITY` to `red`, and 0 to `yellow` and `green`. This will cause all resources to move off the node if any attribute is `red`.
only-green	Assign the value of `-INFINITY` to `red` and `yellow`, and 0 to `green`. This will cause all resources to move off the node if any attribute is `red` or `yellow`.
progressive	Assign the value of the `node-health-red` cluster option to `red`, the value of `node-health-yellow` to `yellow`, and the value of `node-health-green` to `green`. Each node is additionally assigned a score of `node-health-base` (this allows resources to start even if some attributes are `yellow`). This strategy gives the administrator finer control over how important each value is.
custom	Track node health attributes using the same values as `progressive` for `red`, `yellow`, and `green`, but do not take them into account. The administrator is expected to implement a policy by defining Rules referencing node health attributes.

3.3.3. Exempting a Resource from Health Restrictions¶

If you want a resource to be able to run on a node even if its health score would otherwise prevent it, set the resource’s allow-unhealthy-nodes meta-attribute to true (available since 2.1.3).

This is particularly useful for node health agents, to allow them to detect when the node becomes healthy again. If you configure a health agent without this setting, then the health agent will be banned from an unhealthy node, and you will have to investigate and clear the health attribute manually once it is healthy to allow resources on the node again.

If you want the meta-attribute to apply to a clone, it must be set on the clone itself, not on the resource being cloned.

3.3.4. Configuring Node Health Agents¶

Since Pacemaker calculates node health based on node attributes, any method that sets node attributes may be used to measure node health. The most common are resource agents and custom daemons.

Pacemaker provides examples that can be used directly or as a basis for custom code. The ocf:pacemaker:HealthCPU, ocf:pacemaker:HealthIOWait, and ocf:pacemaker:HealthSMART resource agents set node health attributes based on CPU and disk status.

To take advantage of this feature, add the resource to your cluster (generally as a cloned resource with a recurring monitor action, to continually check the health of all nodes). For example:

Example HealthIOWait resource configuration

<clone id="resHealthIOWait-clone">
  <primitive class="ocf" id="HealthIOWait" provider="pacemaker" type="HealthIOWait">
    <instance_attributes id="resHealthIOWait-instance_attributes">
      <nvpair id="resHealthIOWait-instance_attributes-red_limit" name="red_limit" value="30"/>
      <nvpair id="resHealthIOWait-instance_attributes-yellow_limit" name="yellow_limit" value="10"/>
    </instance_attributes>
    <operations>
      <op id="resHealthIOWait-monitor-interval-5" interval="5" name="monitor" timeout="5"/>
      <op id="resHealthIOWait-start-interval-0s" interval="0s" name="start" timeout="10s"/>
      <op id="resHealthIOWait-stop-interval-0s" interval="0s" name="stop" timeout="10s"/>
    </operations>
  </primitive>
</clone>

The resource agents use attrd_updater to set proper status for each node running this resource, as a node attribute whose name starts with #health (for HealthIOWait, the node attribute is named #health-iowait).

When a node is no longer faulty, you can force the cluster to make it available to take resources without waiting for the next monitor, by setting the node health attribute to green. For example:

Force node1 to be marked as healthy

# attrd_updater --name "#health-iowait" --update "green" --node "node1"