source/adminguide/clusters.rst

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Adding Clusters
---------------

Additional clusters can be added at any time to add a new set of hosts. 
For requirements and instructions, see :ref:`adding-a-cluster`.


CloudStack DRS
--------------
DRS is a process that can rebalance instances across hypervisor hosts in a cluster as defined by the algorithm selected. 
There are two algorithms available:

#. **Condensed** spread instances across hosts as densely as possible. 
   This is useful for reducing the number of hosts used.
#. **Balanced** spread instances across hosts as evenly as possible. 
   This is useful for reducing the load on each host. This is the default algorithm.

.. note::
   Deployment planner will not consider DRS while deploying instances.

Configuring DRS
~~~~~~~~~~~~~~~
Following are the configuration parameters for DRS.

.. list-table:: DRS related cluster parameters
   :header-rows: 1

   * - Parameter
     - Default
     - Description
   * - ``drs.plan.expire.interval``
     - `30`
     - The interval in days after which the DRS events will be cleaned up.
   * - ``drs.automatic.enable``
     - `false`
     - Enable/disable automatic DRS on a cluster.
   * - ``drs.automatic.interval``
     - `60`
     - The interval in minutes after which a periodic background thread will schedule DRS for a cluster.
   * - ``drs.max.migrations``
     - `50`
     - Maximum number of instances to be migrated in one DRS execution.
   * - ``drs.algorithm``
     - `condensed`
     - DRS algorithm to be executed on the cluster. Available algorithms - `condensed`, `balanced`.
   * - ``drs.imbalance``
     - `0.4`
     - Percentage (as a value between 0.0 and 1.0) of imbalance allowed in the cluster. 1.0 means no imbalance
       is allowed and 0.0 means imbalance is allowed.
   * - ``drs.metric``
     - `memory`
     - The cluster imbalance metric to use when considering the imbalance in cluster. Possible values are memory, cpu.

.. note::
  Scope of ``drs.plan.expire.interval`` is global and for rest is cluster level.

.. note::
   Very high value for ``drs.max.migrations`` can result in management server using up all of it's workers for DRS tasks
   and not being able to execute other tasks.

Executing manual DRS on a cluster
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DRS can also be manually executed for a cluster.

Follow these steps to execute DRS for a cluster using the UI:

#. Configure DRS parameters as per the requirement.
   |drs-cluster-settings.png|
#. Open DRS tab for the cluster.
   |drs-cluster-tab.png|
#. Select the iteration percentage to execute for the cluster.
#. Click on `Generate DRS Plan` button. A modal will appear showing migrations 
   for that cluster as per the configured algorithm.
   |drs-plan.png|
#. Validate the migrations, and click on `Execute` button to execute the 
   suggested migrations.


.. note:: 
   Only one DRS process can run for a cluster at a time. If you try to run DRS while another 
   DRS process is running, the second process will fail.


.. |drs-cluster-settings.png| image:: /_static/images/drs-cluster-settings.png
   :alt: DRS settings for a cluster.

.. |drs-cluster-tab.png| image:: /_static/images/drs-cluster-tab.png
   :alt: DRS tab for a cluster.

.. |drs-plan.png| image:: /_static/images/drs-plan.png
   :alt: DRS plan for a cluster.