Advanced Configuration ---------------------- .. index:: single: start-delay; operation attribute single: interval-origin; operation attribute single: interval; interval-origin single: operation; interval-origin single: operation; start-delay 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: .. topic:: Example recurring action that runs relative to base date/time .. code-block:: xml .. index:: single: resource; failure recovery single: operation; failure recovery .. _failure-handling: Handling Resource Failure ######################### By default, Pacemaker will attempt to recover failed resources by restarting them. However, failure recovery is highly configurable. .. index:: single: resource; failure count single: operation; failure count 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: .. code-block:: none # 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: .. code-block:: none # 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). .. index:: single: migration-threshold; resource meta-attribute single: resource; migration-threshold 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. [#]_ 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 failure 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. .. index:: single: resource; move Moving Resources ################ 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. .. index:: single: standby mode single: node; standby mode 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: .. code-block:: none # 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: .. code-block:: none # crm_standby -G --node sles-2 To change the current node's standby status, use ``-v`` instead of ``-G``: .. code-block:: none # 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. 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: .. code-block:: none # crm_resource -M -r Email -H sles-2 Behind the scenes, the tool will create the following location constraint: .. code-block:: xml It is important to note that subsequent invocations of ``crm_resource -M`` are not cumulative. So, if you ran these commands: .. code-block:: none # 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: .. code-block:: none # 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``: .. code-block:: none # 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: .. code-block:: none # crm_resource -B -r Email which will instead create a negative constraint, like: .. code-block:: xml 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 :ref:`node-score-equal`. The tool can detect some of these cases and deals with them by creating both positive and negative constraints. For example: .. code-block:: xml which has the same long-term consequences as discussed earlier. Moving Resources Due to Connectivity Changes ____________________________________________ You can configure the cluster to move resources when external connectivity is lost in two steps. .. index:: single: ocf:pacemaker:ping resource single: ping resource 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. .. table:: **Commonly Used ocf:pacemaker:ping Resource Parameters** :widths: 1 4 +--------------------+--------------------------------------------------------------+ | Resource Parameter | Description | +====================+==============================================================+ | dampen | .. index:: | | | single: ocf:pacemaker:ping resource; dampen parameter | | | single: dampen; ocf:pacemaker:ping resource parameter | | | | | | 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 | .. index:: | | | single: ocf:pacemaker:ping resource; multiplier parameter | | | single: multiplier; ocf:pacemaker:ping resource parameter | | | | | | 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 | .. index:: | | | single: ocf:pacemaker:ping resource; host_list parameter | | | single: host_list; ocf:pacemaker:ping resource parameter | | | | | | 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. | +--------------------+--------------------------------------------------------------+ .. topic:: Example ping resource that checks node connectivity once every minute .. code-block:: xml .. 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. Tell Pacemaker How to Interpret the Connectivity Data ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. important:: Before attempting the following, make sure you understand :ref:`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. .. topic:: Don't run a resource on unconnected nodes .. code-block:: xml 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. .. topic:: Run only on nodes connected to three or more ping targets .. code-block:: xml ... ... ... 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``). .. topic:: Prefer node with most connected ping nodes .. code-block:: xml 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: .. topic:: How the cluster translates the above location constraint .. code-block:: xml 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). .. topic:: More complex example of choosing location based on connectivity .. code-block:: xml .. _live-migration: 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 :ref:`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: .. topic:: 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. .. index:: single: reload single: reload-agent 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. .. rubric:: Footnotes .. [#] 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.