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.. _read_balancer:
=======================================
Operating the Read (Primary) Balancer
=======================================
You might be wondering: How can I improve performance in my Ceph cluster?
One important data point you can check is the ``read_balance_score`` on each
of your replicated pools.
This metric, available via ``ceph osd pool ls detail`` (see :ref:`rados_pools`
for more details) indicates read performance, or how balanced the primaries are
for each replicated pool. In most cases, if a ``read_balance_score`` is above 1
(for instance, 1.5), this means that your pool has unbalanced primaries and that
you may want to try improving your read performance with the read balancer.
Online Optimization
===================
At present, there is no online option for the read balancer. However, we plan to add
the read balancer as an option to the :ref:`balancer` in the next Ceph version
so it can be enabled to run automatically in the background like the upmap balancer.
Offline Optimization
====================
Primaries are updated with an offline optimizer that is built into the
:ref:`osdmaptool`.
#. Grab the latest copy of your osdmap:
.. prompt:: bash $
ceph osd getmap -o om
#. Run the optimizer:
.. prompt:: bash $
osdmaptool om --read out.txt --read-pool <pool name> [--vstart]
It is highly recommended that you run the capacity balancer before running the
balancer to ensure optimal results. See :ref:`upmap` for details on how to balance
capacity in a cluster.
#. Apply the changes:
.. prompt:: bash $
source out.txt
In the above example, the proposed changes are written to the output file
``out.txt``. The commands in this procedure are normal Ceph CLI commands
that can be run in order to apply the changes to the cluster.
If you are working in a vstart cluster, you may pass the ``--vstart`` parameter
as shown above so the CLI commands are formatted with the `./bin/` prefix.
Note that any time the number of pgs changes (for instance, if the pg autoscaler [:ref:`pg-autoscaler`]
kicks in), you should consider rechecking the scores and rerunning the balancer if needed.
To see some details about what the tool is doing, you can pass
``--debug-osd 10`` to ``osdmaptool``. To see even more details, pass
``--debug-osd 20`` to ``osdmaptool``.
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