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53 lines
1.8 KiB
ReStructuredText
53 lines
1.8 KiB
ReStructuredText
Cephadm Concepts
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================
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Cephadm Scheduler
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-----------------
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Cephadm uses a declarative state to define the layout of the cluster. This
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state consists of a list of service specifications containing placement
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specifications (See :ref:`orchestrator-cli-service-spec` ).
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Cephadm constantly compares list of actually running daemons in the cluster
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with the desired service specifications and will either add or remove new
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daemons.
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First, cephadm will select a list of candidate hosts. It first looks for
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explicit host names and will select those. In case there are no explicit hosts
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defined, cephadm looks for a label specification. If there is no label defined
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in the specification, cephadm will select hosts based on a host pattern. If
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there is no pattern defined, cepham will finally select all known hosts as
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candidates.
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Then, cephadm will consider existing daemons of this services and will try to
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avoid moving any daemons.
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Cephadm supports the deployment of a specific amount of services. Let's
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consider a service specification like so:
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.. code-block:: yaml
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service_type: mds
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service_name: myfs
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placement:
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count: 3
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label: myfs
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This instructs cephadm to deploy three daemons on hosts labeled with
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``myfs`` across the cluster.
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Then, in case there are less than three daemons deployed on the candidate
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hosts, cephadm will then randomly choose hosts for deploying new daemons.
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In case there are more than three daemons deployed, cephadm will remove
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existing daemons.
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Finally, cephadm will remove daemons on hosts that are outside of the list of
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candidate hosts.
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However, there is a special cases that cephadm needs to consider.
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In case the are fewer hosts selected by the placement specification than
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demanded by ``count``, cephadm will only deploy on selected hosts.
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