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.. _nova-scheduler:

.. list-table:: Description of scheduler configuration options
   :header-rows: 1
   :class: config-ref-table

   * - Configuration option = Default value
     - Description

   * - ``discover_hosts_in_cells_interval`` = ``-1``

     - (Integer) Periodic task interval.

       This value controls how often (in seconds) the scheduler should attempt to discover new hosts that have been added to cells. If negative (the default), no automatic discovery will occur.

       Small deployments may want this periodic task enabled, as surveying the cells for new hosts is likely to be lightweight enough to not cause undue burdon to the scheduler. However, larger clouds (and those that are not adding hosts regularly) will likely want to disable this automatic behavior and instead use the `nova-manage cell_v2 discover_hosts` command when hosts have been added to a cell.

   * - ``driver`` = ``filter_scheduler``

     - (String) The class of the driver used by the scheduler.

       The options are chosen from the entry points under the namespace 'nova.scheduler.driver' in 'setup.cfg'.

       Possible values:

       * A string, where the string corresponds to the class name of a scheduler driver. There are a number of options available: ** 'caching_scheduler', which aggressively caches the system state for better individual scheduler performance at the risk of more retries when running multiple schedulers ** 'chance_scheduler', which simply picks a host at random ** 'fake_scheduler', which is used for testing ** A custom scheduler driver. In this case, you will be responsible for creating and maintaining the entry point in your 'setup.cfg' file

   * - ``periodic_task_interval`` = ``60``

     - (Integer) Periodic task interval.

       This value controls how often (in seconds) to run periodic tasks in the scheduler. The specific tasks that are run for each period are determined by the particular scheduler being used.

       If this is larger than the nova-service 'service_down_time' setting, Nova may report the scheduler service as down. This is because the scheduler driver is responsible for sending a heartbeat and it will only do that as often as this option allows. As each scheduler can work a little differently than the others, be sure to test this with your selected scheduler.

       Possible values:

       * An integer, where the integer corresponds to periodic task interval in seconds. 0 uses the default interval (60 seconds). A negative value disables periodic tasks.

       Related options:

       * ``nova-service service_down_time``

   * - ``host_manager`` = ``host_manager``

     - (String) The scheduler host manager to use.

       The host manager manages the in-memory picture of the hosts that the scheduler uses. The options values are chosen from the entry points under the namespace 'nova.scheduler.host_manager' in 'setup.cfg'.

   * - ``max_attempts`` = ``3``

     - (Integer) Maximum number of schedule attempts for a chosen host.

       This is the maximum number of attempts that will be made to schedule an instance before it is assumed that the failures aren't due to normal occasional race conflicts, but rather some other problem. When this is reached a MaxRetriesExceeded exception is raised, and the instance is set to an error state.

       Possible values:

       * A positive integer, where the integer corresponds to the max number of attempts that can be made when scheduling an instance.