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66 lines
2.2 KiB
ReStructuredText
66 lines
2.2 KiB
ReStructuredText
.. _ceph-volume-systemd:
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systemd
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=======
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As part of the :ref:`ceph-volume-lvm-activate` process, a few systemd units will get enabled
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that will use the OSD id and uuid as part of their name. These units will be
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run when the system boots, and will proceed to activate their corresponding
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volumes.
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The API for activation requires both the :term:`OSD id` and :term:`OSD uuid`,
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which get persisted by systemd. Internally, the activation process enables the
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systemd unit using the following convention::
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ceph-volume@<type>-<extra metadata>
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Where ``type`` is the sub-command used to parse the extra metadata, and ``extra
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metadata`` is any additional information needed by the sub-command to be able
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to activate the OSD. For example an OSD with an ID of 0, for the ``lvm``
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sub-command would look like::
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systemctl enable ceph-volume@lvm-0-0A3E1ED2-DA8A-4F0E-AA95-61DEC71768D6
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Process
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-------
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The systemd unit is a :term:`systemd oneshot` service, meant to start at boot after the
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local filesystem is ready to be used.
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Upon startup, it will identify the logical volume using :term:`LVM tags`,
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finding a matching ID and later ensuring it is the right one with
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the :term:`OSD uuid`.
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After identifying the correct volume it will then proceed to mount it by using
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the OSD destination conventions, that is::
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/var/lib/ceph/osd/<cluster name>-<osd id>
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For our example OSD with an id of ``0``, that means the identified device will
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be mounted at::
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/var/lib/ceph/osd/ceph-0
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Once that process is complete, a call will be made to start the OSD::
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systemctl start ceph-osd@0
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Failure and Retries
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-------------------
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It is common to have failures when a system is coming up online. The devices
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are sometimes not fully available and this unpredictable behavior may cause an
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OSD to not be ready to be used.
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There are two configurable environment variables used to set the retry
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behavior:
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* ``CEPH_VOLUME_SYSTEMD_TRIES``: Defaults to 30
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* ``CEPH_VOLUME_SYSTEMD_INTERVAL``: Defaults to 5
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The *"tries"* is a number that sets the maximum amount of times the unit will
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attempt to activate an OSD before giving up.
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The *"interval"* is a value in seconds that determines the waiting time before
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initiating another try at activating the OSD.
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