9. Advanced Configuration

9.1. Specifying When Recurring Actions are Performed

By default, recurring actions are scheduled relative to when the resource started. In some cases, you might prefer that a recurring action start relative to a specific date and time. For example, you might schedule an in-depth monitor to run once every 24 hours, and want it to run outside business hours.

To do this, set the operation’s interval-origin. The cluster uses this point to calculate the correct start-delay such that the operation will occur at interval-origin plus a multiple of the operation interval.

For example, if the recurring operation’s interval is 24h, its interval-origin is set to 02:00, and it is currently 14:32, then the cluster would initiate the operation after 11 hours and 28 minutes.

The value specified for interval and interval-origin can be any date/time conforming to the ISO8601 standard. By way of example, to specify an operation that would run on the first Monday of 2021 and every Monday after that, you would add:

Example recurring action that runs relative to base date/time

<op id="intensive-monitor" name="monitor" interval="P7D" interval-origin="2021-W01-1"/>

9.2. Handling Resource Failure

By default, Pacemaker will attempt to recover failed resources by restarting them. However, failure recovery is highly configurable.

9.2.1. Failure Counts

Pacemaker tracks resource failures for each combination of node, resource, and operation (start, stop, monitor, etc.).

You can query the fail count for a particular node, resource, and/or operation using the crm_failcount command. For example, to see how many times the 10-second monitor for myrsc has failed on node1, run:

# crm_failcount --query -r myrsc -N node1 -n monitor -I 10s

If you omit the node, crm_failcount will use the local node. If you omit the operation and interval, crm_failcount will display the sum of the fail counts for all operations on the resource.

You can use crm_resource --cleanup or crm_failcount --delete to clear fail counts. For example, to clear the above monitor failures, run:

# crm_resource --cleanup -r myrsc -N node1 -n monitor -I 10s

If you omit the resource, crm_resource --cleanup will clear failures for all resources. If you omit the node, it will clear failures on all nodes. If you omit the operation and interval, it will clear the failures for all operations on the resource.

Note

Even when cleaning up only a single operation, all failed operations will disappear from the status display. This allows us to trigger a re-check of the resource’s current status.

Higher-level tools may provide other commands for querying and clearing fail counts.

The crm_mon tool shows the current cluster status, including any failed operations. To see the current fail counts for any failed resources, call crm_mon with the --failcounts option. This shows the fail counts per resource (that is, the sum of any operation fail counts for the resource).

9.2.2. Failure Response

Normally, if a running resource fails, pacemaker will try to stop it and start it again. Pacemaker will choose the best location to start it each time, which may be the same node that it failed on.

However, if a resource fails repeatedly, it is possible that there is an underlying problem on that node, and you might desire trying a different node in such a case. Pacemaker allows you to set your preference via the migration-threshold resource meta-attribute. [1]

If you define migration-threshold to N for a resource, it will be banned from the original node after N failures there.

Note

The migration-threshold is per resource, even though fail counts are tracked per operation. The operation fail counts are added together to compare against the migration-threshold.

By default, fail counts remain until manually cleared by an administrator using crm_resource --cleanup or crm_failcount --delete (hopefully after first fixing the failure’s cause). It is possible to have fail counts expire automatically by setting the failure-timeout resource meta-attribute.

Important

A successful operation does not clear past failures. If a recurring monitor operation fails once, succeeds many times, then fails again days later, its fail count is 2. Fail counts are cleared only by manual intervention or falure timeout.

For example, setting migration-threshold to 2 and failure-timeout to 60s would cause the resource to move to a new node after 2 failures, and allow it to move back (depending on stickiness and constraint scores) after one minute.

Note

failure-timeout is measured since the most recent failure. That is, older failures do not individually time out and lower the fail count. Instead, all failures are timed out simultaneously (and the fail count is reset to 0) if there is no new failure for the timeout period.

There are two exceptions to the migration threshold: when a resource either fails to start or fails to stop.

If the cluster property start-failure-is-fatal is set to true (which is the default), start failures cause the fail count to be set to INFINITY and thus always cause the resource to move immediately.

Stop failures are slightly different and crucial. If a resource fails to stop and fencing is enabled, then the cluster will fence the node in order to be able to start the resource elsewhere. If fencing is disabled, then the cluster has no way to continue and will not try to start the resource elsewhere, but will try to stop it again after any failure timeout or clearing.

9.3. Moving Resources

9.3.1. Moving Resources Manually

There are primarily two occasions when you would want to move a resource from its current location: when the whole node is under maintenance, and when a single resource needs to be moved.

9.3.1.1. Standby Mode

Since everything eventually comes down to a score, you could create constraints for every resource to prevent them from running on one node. While Pacemaker configuration can seem convoluted at times, not even we would require this of administrators.

Instead, you can set a special node attribute which tells the cluster “don’t let anything run here”. There is even a helpful tool to help query and set it, called crm_standby. To check the standby status of the current machine, run:

# crm_standby -G

A value of on indicates that the node is not able to host any resources, while a value of off says that it can.

You can also check the status of other nodes in the cluster by specifying the –node option:

# crm_standby -G --node sles-2

To change the current node’s standby status, use -v instead of -G:

# crm_standby -v on

Again, you can change another host’s value by supplying a hostname with --node.

A cluster node in standby mode will not run resources, but still contributes to quorum, and may fence or be fenced by nodes.

9.3.1.2. Moving One Resource

When only one resource is required to move, we could do this by creating location constraints. However, once again we provide a user-friendly shortcut as part of the crm_resource command, which creates and modifies the extra constraints for you. If Email were running on sles-1 and you wanted it moved to a specific location, the command would look something like:

# crm_resource -M -r Email -H sles-2

Behind the scenes, the tool will create the following location constraint:

<rsc_location id="cli-prefer-Email" rsc="Email" node="sles-2" score="INFINITY"/>

It is important to note that subsequent invocations of crm_resource -M are not cumulative. So, if you ran these commands:

# crm_resource -M -r Email -H sles-2
# crm_resource -M -r Email -H sles-3

then it is as if you had never performed the first command.

To allow the resource to move back again, use:

# crm_resource -U -r Email

Note the use of the word allow. The resource can move back to its original location, but depending on resource-stickiness, location constraints, and so forth, it might stay where it is.

To be absolutely certain that it moves back to sles-1, move it there before issuing the call to crm_resource -U:

# crm_resource -M -r Email -H sles-1
# crm_resource -U -r Email

Alternatively, if you only care that the resource should be moved from its current location, try:

# crm_resource -B -r Email

which will instead create a negative constraint, like:

<rsc_location id="cli-ban-Email-on-sles-1" rsc="Email" node="sles-1" score="-INFINITY"/>

This will achieve the desired effect, but will also have long-term consequences. As the tool will warn you, the creation of a -INFINITY constraint will prevent the resource from running on that node until crm_resource -U is used. This includes the situation where every other cluster node is no longer available!

In some cases, such as when resource-stickiness is set to INFINITY, it is possible that you will end up with the problem described in What if Two Nodes Have the Same Score. The tool can detect some of these cases and deals with them by creating both positive and negative constraints. For example:

<rsc_location id="cli-ban-Email-on-sles-1" rsc="Email" node="sles-1" score="-INFINITY"/>
<rsc_location id="cli-prefer-Email" rsc="Email" node="sles-2" score="INFINITY"/>

which has the same long-term consequences as discussed earlier.

9.3.2. Moving Resources Due to Connectivity Changes

You can configure the cluster to move resources when external connectivity is lost in two steps.

9.3.2.1. Tell Pacemaker to Monitor Connectivity

First, add an ocf:pacemaker:ping resource to the cluster. The ping resource uses the system utility of the same name to a test whether a list of machines (specified by DNS hostname or IP address) are reachable, and uses the results to maintain a node attribute.

The node attribute is called pingd by default, but is customizable in order to allow multiple ping groups to be defined.

Normally, the ping resource should run on all cluster nodes, which means that you’ll need to create a clone. A template for this can be found below, along with a description of the most interesting parameters.

Commonly Used ocf:pacemaker:ping Resource Parameters

Resource Parameter	Description
dampen	The time to wait (dampening) for further changes to occur. Use this to prevent a resource from bouncing around the cluster when cluster nodes notice the loss of connectivity at slightly different times.
multiplier	The number of connected ping nodes gets multiplied by this value to get a score. Useful when there are multiple ping nodes configured.
host_list	The machines to contact in order to determine the current connectivity status. Allowed values include resolvable DNS connectivity host names, IPv4 addresses, and IPv6 addresses.

Example ping resource that checks node connectivity once every minute

<clone id="Connected">
   <primitive id="ping" class="ocf" provider="pacemaker" type="ping">
    <instance_attributes id="ping-attrs">
      <nvpair id="ping-dampen"     name="dampen" value="5s"/>
      <nvpair id="ping-multiplier" name="multiplier" value="1000"/>
      <nvpair id="ping-hosts"      name="host_list" value="my.gateway.com www.bigcorp.com"/>
    </instance_attributes>
    <operations>
      <op id="ping-monitor-60s" interval="60s" name="monitor"/>
    </operations>
   </primitive>
</clone>

Important

You’re only half done. The next section deals with telling Pacemaker how to deal with the connectivity status that ocf:pacemaker:ping is recording.

9.3.2.2. Tell Pacemaker How to Interpret the Connectivity Data

Important

Before attempting the following, make sure you understand Rules.

There are a number of ways to use the connectivity data.

The most common setup is for people to have a single ping target (for example, the service network’s default gateway), to prevent the cluster from running a resource on any unconnected node.

Don’t run a resource on unconnected nodes

<rsc_location id="WebServer-no-connectivity" rsc="Webserver">
   <rule id="ping-exclude-rule" score="-INFINITY" >
      <expression id="ping-exclude" attribute="pingd" operation="not_defined"/>
   </rule>
</rsc_location>

A more complex setup is to have a number of ping targets configured. You can require the cluster to only run resources on nodes that can connect to all (or a minimum subset) of them.

Run only on nodes connected to three or more ping targets

<primitive id="ping" provider="pacemaker" class="ocf" type="ping">
... <!-- omitting some configuration to highlight important parts -->
   <nvpair id="ping-multiplier" name="multiplier" value="1000"/>
...
</primitive>
...
<rsc_location id="WebServer-connectivity" rsc="Webserver">
   <rule id="ping-prefer-rule" score="-INFINITY" >
      <expression id="ping-prefer" attribute="pingd" operation="lt" value="3000"/>
   </rule>
</rsc_location>

Alternatively, you can tell the cluster only to prefer nodes with the best connectivity, by using score-attribute in the rule. Just be sure to set multiplier to a value higher than that of resource-stickiness (and don’t set either of them to INFINITY).

Prefer node with most connected ping nodes

<rsc_location id="WebServer-connectivity" rsc="Webserver">
   <rule id="ping-prefer-rule" score-attribute="pingd" >
      <expression id="ping-prefer" attribute="pingd" operation="defined"/>
   </rule>
</rsc_location>

It is perhaps easier to think of this in terms of the simple constraints that the cluster translates it into. For example, if sles-1 is connected to all five ping nodes but sles-2 is only connected to two, then it would be as if you instead had the following constraints in your configuration:

How the cluster translates the above location constraint

<rsc_location id="ping-1" rsc="Webserver" node="sles-1" score="5000"/>
<rsc_location id="ping-2" rsc="Webserver" node="sles-2" score="2000"/>

The advantage is that you don’t have to manually update any constraints whenever your network connectivity changes.

You can also combine the concepts above into something even more complex. The example below shows how you can prefer the node with the most connected ping nodes provided they have connectivity to at least three (again assuming that multiplier is set to 1000).

More complex example of choosing location based on connectivity

<rsc_location id="WebServer-connectivity" rsc="Webserver">
   <rule id="ping-exclude-rule" score="-INFINITY" >
      <expression id="ping-exclude" attribute="pingd" operation="lt" value="3000"/>
   </rule>
   <rule id="ping-prefer-rule" score-attribute="pingd" >
      <expression id="ping-prefer" attribute="pingd" operation="defined"/>
   </rule>
</rsc_location>

9.3.3. Migrating Resources

Normally, when the cluster needs to move a resource, it fully restarts the resource (that is, it stops the resource on the current node and starts it on the new node).

However, some types of resources, such as many virtual machines, are able to move to another location without loss of state (often referred to as live migration or hot migration). In pacemaker, this is called resource migration. Pacemaker can be configured to migrate a resource when moving it, rather than restarting it.

Not all resources are able to migrate; see the migration checklist below. Even those that can, won’t do so in all situations. Conceptually, there are two requirements from which the other prerequisites follow:

• The resource must be active and healthy at the old location; and
• everything required for the resource to run must be available on both the old and new locations.

The cluster is able to accommodate both push and pull migration models by requiring the resource agent to support two special actions: migrate_to (performed on the current location) and migrate_from (performed on the destination).

In push migration, the process on the current location transfers the resource to the new location where is it later activated. In this scenario, most of the work would be done in the migrate_to action and, if anything, the activation would occur during migrate_from.

Conversely for pull, the migrate_to action is practically empty and migrate_from does most of the work, extracting the relevant resource state from the old location and activating it.

There is no wrong or right way for a resource agent to implement migration, as long as it works.

Migration Checklist

• The resource may not be a clone.
• The resource agent standard must be OCF.
• The resource must not be in a failed or degraded state.
• The resource agent must support migrate_to and migrate_from actions, and advertise them in its meta-data.
• The resource must have the allow-migrate meta-attribute set to true (which is not the default).

If an otherwise migratable resource depends on another resource via an ordering constraint, there are special situations in which it will be restarted rather than migrated.

For example, if the resource depends on a clone, and at the time the resource needs to be moved, the clone has instances that are stopping and instances that are starting, then the resource will be restarted. The scheduler is not yet able to model this situation correctly and so takes the safer (if less optimal) path.

Also, if a migratable resource depends on a non-migratable resource, and both need to be moved, the migratable resource will be restarted.

9.4. Reloading an Agent After a Definition Change

The cluster automatically detects changes to the configuration of active resources. The cluster’s normal response is to stop the service (using the old definition) and start it again (with the new definition). This works, but some resource agents are smarter and can be told to use a new set of options without restarting.

To take advantage of this capability, the resource agent must:

• Implement the reload-agent action. What it should do depends completely on your application!

  Note

  Resource agents may also implement a reload action to make the managed service reload its own native configuration. This is different from reload-agent, which makes effective changes in the resource’s Pacemaker configuration (specifically, the values of the agent’s reloadable parameters).

• Advertise the reload-agent operation in the actions section of its meta-data.

• Set the reloadable attribute to 1 in the parameters section of its meta-data for any parameters eligible to be reloaded after a change.

Once these requirements are satisfied, the cluster will automatically know to reload the resource (instead of restarting) when a reloadable parameter changes.

Note

Metadata will not be re-read unless the resource needs to be started. If you edit the agent of an already active resource to set a parameter reloadable, the resource may restart the first time the parameter value changes.

Note

If both a reloadable and non-reloadable parameter are changed simultaneously, the resource will be restarted.

Footnotes

[1]	The naming of this option was perhaps unfortunate as it is easily confused with live migration, the process of moving a resource from one node to another without stopping it. Xen virtual guests are the most common example of resources that can be migrated in this manner.