diff --git a/doc/sphinx/Pacemaker_Explained/operations.rst b/doc/sphinx/Pacemaker_Explained/operations.rst index c831f815c6..a8df8f14d9 100644 --- a/doc/sphinx/Pacemaker_Explained/operations.rst +++ b/doc/sphinx/Pacemaker_Explained/operations.rst @@ -1,670 +1,671 @@ .. index:: single: resource; action single: resource; operation .. _operation: Resource Operations ------------------- *Operations* are actions the cluster can perform on a resource by calling the resource agent. Resource agents must support certain common operations such as start, stop, and monitor, and may implement any others. Operations may be explicitly configured for two purposes: to override defaults for options (such as timeout) that the cluster will use whenever it initiates the operation, and to run an operation on a recurring basis (for example, to monitor the resource for failure). .. topic:: An OCF resource with a non-default start timeout .. code-block:: xml Pacemaker identifies operations by a combination of name and interval, so this combination must be unique for each resource. That is, you should not configure two operations for the same resource with the same name and interval. .. _operation_properties: Operation Properties #################### The ``id``, ``name``, ``interval``, and ``role`` operation properties may be specified only as XML attributes of the ``op`` element. Other operation properties may be specified in any of the following ways, from highest precedence to lowest: * directly in the ``op`` element as an XML attribute * in an ``nvpair`` element within a ``meta_attributes`` element within the ``op`` element * in an ``nvpair`` element within a ``meta_attributes`` element within :ref:`operation defaults ` If not specified, the default from the table below is used. .. list-table:: **Operation Properties** :class: longtable :widths: 2 2 3 4 :header-rows: 1 * - Name - Type - Default - Description * - .. _op_id: .. index:: pair: op; id single: id; action property single: action; property, id id - :ref:`id ` - - A unique identifier for the XML element *(required)* * - .. _op_name: .. index:: pair: op; name single: name; action property single: action; property, name name - :ref:`text ` - - An action name supported by the resource agent *(required)* * - .. _op_interval: .. index:: pair: op; interval single: interval; action property single: action; property, interval interval - :ref:`duration ` - 0 - If this is a positive value, Pacemaker will schedule recurring instances of this operation at the given interval (which makes sense only with :ref:`name ` set to :ref:`monitor `). If this is 0, Pacemaker will apply other properties configured for this operation to instances that are scheduled as needed during normal cluster operation. *(required)* * - .. _op_role: .. index:: pair: op; role single: role; action property single: action; property, role role - :ref:`enumeration ` - - If this is set, the operation configuration applies only on nodes where the cluster expects the resource to be in the specified role. This makes sense only for recurring monitors. Allowed values: ``Started``, ``Stopped``, and in the case of :ref:`promotable clone resources `, ``Unpromoted`` and ``Promoted``. * - .. _op_timeout: .. index:: pair: op; timeout single: timeout; action property single: action; property, timeout timeout - :ref:`timeout ` - 20s - If resource agent execution does not complete within this amount of time, the action will be considered failed. **Note:** timeouts for fencing agents are handled specially (see the :ref:`fencing` chapter). * - .. _op_on_fail: .. index:: pair: op; on-fail single: on-fail; action property single: action; property, on-fail on-fail - :ref:`enumeration ` - * If ``name`` is ``stop``: ``fence`` if :ref:`stonith-enabled ` is true, otherwise ``block`` * If ``name`` is ``demote``: ``on-fail`` of the ``monitor`` action with ``role`` set to ``Promoted``, if present, enabled, and configured to a value other than ``demote``, or ``restart`` otherwise * Otherwise: ``restart`` - How the cluster should respond to a failure of this action. Allowed values: * ``ignore:`` Pretend the resource did not fail * ``block:`` Do not perform any further operations on the resource * ``stop:`` Stop the resource and leave it stopped * ``demote:`` Demote the resource, without a full restart. This is valid only for ``promote`` actions, and for ``monitor`` actions with both a nonzero ``interval`` and ``role`` set to ``Promoted``; for any other action, a configuration error will be logged, and the default behavior will be used. *(since 2.0.5)* * ``restart:`` Stop the resource, and start it again if allowed (possibly on a different node) * ``fence:`` Fence the node on which the resource failed * ``standby:`` Put the node on which the resource failed in standby mode (forcing *all* resources away) * - .. _op_enabled: .. index:: pair: op; enabled single: enabled; action property single: action; property, enabled enabled - :ref:`boolean ` - true - If ``false``, ignore this operation definition. This does not suppress all actions of this type, but is typically used to pause a recurring monitor. This can complement the resource being unmanaged (:ref:`is-managed ` set to ``false``), which does not stop recurring operations. Maintenance mode, which does stop configured monitors, overrides this setting. * - .. _op_record_pending: .. index:: pair: op; record-pending single: record-pending; action property single: action; property, record-pending record-pending - :ref:`boolean ` - true - Operation results are always recorded when the operation completes (successful or not). If this is ``true``, operations will also be recorded when initiated, so that status output can indicate that the operation is in progress. .. note:: Only one action can be configured for any given combination of ``name`` and ``interval``. .. note:: When ``on-fail`` is set to ``demote``, recovery from failure by a successful demote causes the cluster to recalculate whether and where a new instance should be promoted. The node with the failure is eligible, so if promotion scores have not changed, it will be promoted again. There is no direct equivalent of ``migration-threshold`` for the promoted role, but the same effect can be achieved with a location constraint using a :ref:`rule ` with a node attribute expression for the resource's fail count. For example, to immediately ban the promoted role from a node with any failed promote or promoted instance monitor: .. code-block:: xml This example assumes that there is a promotable clone of the ``my_primitive`` resource (note that the primitive name, not the clone name, is used in the rule), and that there is a recurring 10-second-interval monitor configured for the promoted role (fail count attributes specify the interval in milliseconds). .. _s-resource-monitoring: Monitoring Resources for Failure ################################ When Pacemaker first starts a resource, it runs one-time ``monitor`` operations (referred to as *probes*) to ensure the resource is running where it's supposed to be, and not running where it's not supposed to be. (This behavior can be affected by the ``resource-discovery`` location constraint property.) Other than those initial probes, Pacemaker will *not* (by default) check that the resource continues to stay healthy [#]_. You must configure ``monitor`` operations explicitly to perform these checks. .. topic:: An OCF resource with a recurring health check .. code-block:: xml By default, a ``monitor`` operation will ensure that the resource is running where it is supposed to. The ``target-role`` property can be used for further checking. For example, if a resource has one ``monitor`` operation with ``interval=10 role=Started`` and a second ``monitor`` operation with ``interval=11 role=Stopped``, the cluster will run the first monitor on any nodes it thinks *should* be running the resource, and the second monitor on any nodes that it thinks *should not* be running the resource (for the truly paranoid, who want to know when an administrator manually starts a service by mistake). .. note:: Currently, monitors with ``role=Stopped`` are not implemented for :ref:`clone ` resources. .. _s-operation-defaults: Setting Global Defaults for Operations ###################################### You can change the global default values for operation properties in a given cluster. These are defined in an ``op_defaults`` section of the CIB's ``configuration`` section, and can be set with ``crm_attribute``. For example, .. code-block:: none # crm_attribute --type op_defaults --name timeout --update 20s would default each operation's ``timeout`` to 20 seconds. If an operation's definition also includes a value for ``timeout``, then that value would be used for that operation instead. When Implicit Operations Take a Long Time ######################################### The cluster will always perform a number of implicit operations: ``start``, ``stop`` and a non-recurring ``monitor`` operation used at startup to check whether the resource is already active. If one of these is taking too long, then you can create an entry for them and specify a longer timeout. .. topic:: An OCF resource with custom timeouts for its implicit actions .. code-block:: xml Multiple Monitor Operations ########################### Provided no two operations (for a single resource) have the same name and interval, you can have as many ``monitor`` operations as you like. In this way, you can do a superficial health check every minute and progressively more intense ones at higher intervals. To tell the resource agent what kind of check to perform, you need to provide each monitor with a different value for a common parameter. The OCF standard creates a special parameter called ``OCF_CHECK_LEVEL`` for this purpose and dictates that it is "made available to the resource agent without the normal ``OCF_RESKEY`` prefix". Whatever name you choose, you can specify it by adding an ``instance_attributes`` block to the ``op`` tag. It is up to each resource agent to look for the parameter and decide how to use it. .. topic:: An OCF resource with two recurring health checks, performing different levels of checks specified via ``OCF_CHECK_LEVEL``. .. code-block:: xml Disabling a Monitor Operation ############################# The easiest way to stop a recurring monitor is to just delete it. However, there can be times when you only want to disable it temporarily. In such cases, simply add ``enabled=false`` to the operation's definition. .. topic:: Example of an OCF resource with a disabled health check .. code-block:: xml This can be achieved from the command line by executing: .. code-block:: none # cibadmin --modify --xml-text '' Once you've done whatever you needed to do, you can then re-enable it with .. code-block:: none # cibadmin --modify --xml-text '' .. 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: 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. .. _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 live 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. + .. rubric:: Footnotes .. [#] Currently, anyway. Automatic monitoring operations may be added in a future version of Pacemaker. .. [#] 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.