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4.3 KiB
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121 lines
4.3 KiB
ReStructuredText
Cephadm Concepts
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================
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.. _cephadm-fqdn:
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Fully qualified domain names vs bare host names
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-----------------------------------------------
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cephadm has very minimal requirements when it comes to resolving host
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names etc. When cephadm initiates an ssh connection to a remote host,
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the host name can be resolved in four different ways:
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- a custom ssh config resolving the name to an IP
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- via an externally maintained ``/etc/hosts``
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- via explicitly providing an IP address to cephadm: ``ceph orch host add <hostname> <IP>``
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- automatic name resolution via DNS.
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Ceph itself uses the command ``hostname`` to determine the name of the
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current host.
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.. note::
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cephadm demands that the name of the host given via ``ceph orch host add``
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equals the output of ``hostname`` on remote hosts.
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Otherwise cephadm can't be sure, the host names returned by
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``ceph * metadata`` match the hosts known to cephadm. This might result
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in a :ref:`cephadm-stray-host` warning.
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When configuring new hosts, there are two **valid** ways to set the
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``hostname`` of a host:
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1. Using the bare host name. In this case:
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- ``hostname`` returns the bare host name.
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- ``hostname -f`` returns the FQDN.
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2. Using the fully qualified domain name as the host name. In this case:
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- ``hostname`` returns the FQDN
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- ``hostname -s`` return the bare host name
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Note that ``man hostname`` recommends ``hostname`` to return the bare
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host name:
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The FQDN (Fully Qualified Domain Name) of the system is the
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name that the resolver(3) returns for the host name, such as,
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ursula.example.com. It is usually the hostname followed by the DNS
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domain name (the part after the first dot). You can check the FQDN
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using ``hostname --fqdn`` or the domain name using ``dnsdomainname``.
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.. code-block:: none
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You cannot change the FQDN with hostname or dnsdomainname.
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The recommended method of setting the FQDN is to make the hostname
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be an alias for the fully qualified name using /etc/hosts, DNS, or
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NIS. For example, if the hostname was "ursula", one might have
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a line in /etc/hosts which reads
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127.0.1.1 ursula.example.com ursula
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Which means, ``man hostname`` recommends ``hostname`` to return the bare
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host name. This in turn means that Ceph will return the bare host names
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when executing ``ceph * metadata``. This in turn means cephadm also
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requires the bare host name when adding a host to the cluster:
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``ceph orch host add <bare-name>``.
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..
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TODO: This chapter needs to provide way for users to configure
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Grafana in the dashboard, as this is right no very hard to do.
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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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