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+.. _ceph-volume-lvm-prepare:
+
+``prepare``
+===========
+Before you run ``ceph-volume lvm prepare``, we recommend that you provision a
+logical volume. Then you can run ``prepare`` on that logical volume.
+
+``prepare`` adds metadata to logical volumes but does not alter them in any
+other way.
+
+.. note:: This is part of a two-step process to deploy an OSD. If you prefer
+ to deploy an OSD by using only one command, see :ref:`ceph-volume-lvm-create`.
+
+``prepare`` uses :term:`LVM tags` to assign several pieces of metadata to a
+logical volume. Volumes tagged in this way are easier to identify and easier to
+use with Ceph. :term:`LVM tags` identify logical volumes by the role that they
+play in the Ceph cluster (for example: BlueStore data or BlueStore WAL+DB).
+
+:term:`BlueStore<bluestore>` is the default backend. Ceph permits changing
+the backend, which can be done by using the following flags and arguments:
+
+* :ref:`--filestore <ceph-volume-lvm-prepare_filestore>`
+* :ref:`--bluestore <ceph-volume-lvm-prepare_bluestore>`
+
+.. _ceph-volume-lvm-prepare_bluestore:
+
+``bluestore``
+-------------
+:term:`Bluestore<bluestore>` is the default backend for new OSDs. It
+offers more flexibility for devices than :term:`filestore` does. Bluestore
+supports the following configurations:
+
+* a block device, a block.wal device, and a block.db device
+* a block device and a block.wal device
+* a block device and a block.db device
+* a single block device
+
+The ``bluestore`` subcommand accepts physical block devices, partitions on physical
+block devices, or logical volumes as arguments for the various device
+parameters. If a physical block device is provided, a logical volume will be
+created. If the provided volume group's name begins with `ceph`, it will be
+created if it does not yet exist and it will be clobbered and reused if it
+already exists. This allows for a simpler approach to using LVM but at the
+cost of flexibility: no option or configuration can be used to change how the
+logical volume is created.
+
+The ``block`` is specified with the ``--data`` flag, and in its simplest use
+case it looks like:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --bluestore --data vg/lv
+
+A raw device can be specified in the same way:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --bluestore --data /path/to/device
+
+For enabling :ref:`encryption <ceph-volume-lvm-encryption>`, the ``--dmcrypt`` flag is required:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --bluestore --dmcrypt --data vg/lv
+
+If a ``block.db`` device or a ``block.wal`` device is needed, it can be
+specified with ``--block.db`` or ``--block.wal``. These can be physical
+devices, partitions, or logical volumes. ``block.db`` and ``block.wal`` are
+optional for bluestore.
+
+For both ``block.db`` and ``block.wal``, partitions can be used as-is, and
+therefore are not made into logical volumes.
+
+While creating the OSD directory, the process uses a ``tmpfs`` mount to hold
+the files needed for the OSD. These files are created by ``ceph-osd --mkfs``
+and are ephemeral.
+
+A symlink is created for the ``block`` device, and is optional for ``block.db``
+and ``block.wal``. For a cluster with a default name and an OSD ID of 0, the
+directory looks like this::
+
+ # ls -l /var/lib/ceph/osd/ceph-0
+ lrwxrwxrwx. 1 ceph ceph 93 Oct 20 13:05 block -> /dev/ceph-be2b6fbd-bcf2-4c51-b35d-a35a162a02f0/osd-block-25cf0a05-2bc6-44ef-9137-79d65bd7ad62
+ lrwxrwxrwx. 1 ceph ceph 93 Oct 20 13:05 block.db -> /dev/sda1
+ lrwxrwxrwx. 1 ceph ceph 93 Oct 20 13:05 block.wal -> /dev/ceph/osd-wal-0
+ -rw-------. 1 ceph ceph 37 Oct 20 13:05 ceph_fsid
+ -rw-------. 1 ceph ceph 37 Oct 20 13:05 fsid
+ -rw-------. 1 ceph ceph 55 Oct 20 13:05 keyring
+ -rw-------. 1 ceph ceph 6 Oct 20 13:05 ready
+ -rw-------. 1 ceph ceph 10 Oct 20 13:05 type
+ -rw-------. 1 ceph ceph 2 Oct 20 13:05 whoami
+
+In the above case, a device was used for ``block``, so ``ceph-volume`` created
+a volume group and a logical volume using the following conventions:
+
+* volume group name: ``ceph-{cluster fsid}`` (or if the volume group already
+ exists: ``ceph-{random uuid}``)
+
+* logical volume name: ``osd-block-{osd_fsid}``
+
+
+.. _ceph-volume-lvm-prepare_filestore:
+
+``filestore``
+-------------
+``Filestore<filestore>`` is the OSD backend that prepares logical volumes for a
+:term:`filestore`-backed object-store OSD.
+
+
+``Filestore<filestore>`` uses a logical volume to store OSD data and it uses
+physical devices, partitions, or logical volumes to store the journal. If a
+physical device is used to create a filestore backend, a logical volume will be
+created on that physical device. If the provided volume group's name begins
+with `ceph`, it will be created if it does not yet exist and it will be
+clobbered and reused if it already exists. No special preparation is needed for
+these volumes, but be sure to meet the minimum size requirements for OSD data and
+for the journal.
+
+Use the following command to create a basic filestore OSD:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --filestore --data <data block device>
+
+Use this command to deploy filestore with an external journal:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --filestore --data <data block device> --journal <journal block device>
+
+Use this command to enable :ref:`encryption <ceph-volume-lvm-encryption>`, and note that the ``--dmcrypt`` flag is required:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --filestore --dmcrypt --data <data block device> --journal <journal block device>
+
+The data block device and the journal can each take one of three forms:
+
+* a physical block device
+* a partition on a physical block device
+* a logical volume
+
+If you use a logical volume to deploy filestore, the value that you pass in the
+command *must* be of the format ``volume_group/logical_volume_name``. Since logical
+volume names are not enforced for uniqueness, using this format is an important
+safeguard against accidentally choosing the wrong volume (and clobbering its data).
+
+If you use a partition to deploy filestore, the partition *must* contain a
+``PARTUUID`` that can be discovered by ``blkid``. This ensures that the
+partition can be identified correctly regardless of the device's name (or path).
+
+For example, to use a logical volume for OSD data and a partition
+(``/dev/sdc1``) for the journal, run a command of this form:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --filestore --data volume_group/logical_volume_name --journal /dev/sdc1
+
+Or, to use a bare device for data and a logical volume for the journal:
+
+.. prompt:: bash #
+
+ ceph-volume lvm prepare --filestore --data /dev/sdc --journal volume_group/journal_lv
+
+A generated UUID is used when asking the cluster for a new OSD. These two
+pieces of information (the OSD ID and the OSD UUID) are necessary for
+identifying a given OSD and will later be used throughout the
+:ref:`activation<ceph-volume-lvm-activate>` process.
+
+The OSD data directory is created using the following convention::
+
+ /var/lib/ceph/osd/<cluster name>-<osd id>
+
+To link the journal volume to the mounted data volume, use this command:
+
+.. prompt:: bash #
+
+ ln -s /path/to/journal /var/lib/ceph/osd/<cluster_name>-<osd-id>/journal
+
+To fetch the monmap by using the bootstrap key from the OSD, use this command:
+
+.. prompt:: bash #
+
+ /usr/bin/ceph --cluster ceph --name client.bootstrap-osd --keyring
+ /var/lib/ceph/bootstrap-osd/ceph.keyring mon getmap -o
+ /var/lib/ceph/osd/<cluster name>-<osd id>/activate.monmap
+
+To populate the OSD directory (which has already been mounted), use this ``ceph-osd`` command:
+.. prompt:: bash #
+
+ ceph-osd --cluster ceph --mkfs --mkkey -i <osd id> \ --monmap
+ /var/lib/ceph/osd/<cluster name>-<osd id>/activate.monmap --osd-data \
+ /var/lib/ceph/osd/<cluster name>-<osd id> --osd-journal
+ /var/lib/ceph/osd/<cluster name>-<osd id>/journal \ --osd-uuid <osd uuid>
+ --keyring /var/lib/ceph/osd/<cluster name>-<osd id>/keyring \ --setuser ceph
+ --setgroup ceph
+
+All of the information from the previous steps is used in the above command.
+
+
+
+.. _ceph-volume-lvm-partitions:
+
+Partitioning
+------------
+``ceph-volume lvm`` does not currently create partitions from a whole device.
+If using device partitions the only requirement is that they contain the
+``PARTUUID`` and that it is discoverable by ``blkid``. Both ``fdisk`` and
+``parted`` will create that automatically for a new partition.
+
+For example, using a new, unformatted drive (``/dev/sdd`` in this case) we can
+use ``parted`` to create a new partition. First we list the device
+information::
+
+ $ parted --script /dev/sdd print
+ Model: VBOX HARDDISK (scsi)
+ Disk /dev/sdd: 11.5GB
+ Sector size (logical/physical): 512B/512B
+ Disk Flags:
+
+This device is not even labeled yet, so we can use ``parted`` to create
+a ``gpt`` label before we create a partition, and verify again with ``parted
+print``::
+
+ $ parted --script /dev/sdd mklabel gpt
+ $ parted --script /dev/sdd print
+ Model: VBOX HARDDISK (scsi)
+ Disk /dev/sdd: 11.5GB
+ Sector size (logical/physical): 512B/512B
+ Partition Table: gpt
+ Disk Flags:
+
+Now lets create a single partition, and verify later if ``blkid`` can find
+a ``PARTUUID`` that is needed by ``ceph-volume``::
+
+ $ parted --script /dev/sdd mkpart primary 1 100%
+ $ blkid /dev/sdd1
+ /dev/sdd1: PARTLABEL="primary" PARTUUID="16399d72-1e1f-467d-96ee-6fe371a7d0d4"
+
+
+.. _ceph-volume-lvm-existing-osds:
+
+Existing OSDs
+-------------
+For existing clusters that want to use this new system and have OSDs that are
+already running there are a few things to take into account:
+
+.. warning:: this process will forcefully format the data device, destroying
+ existing data, if any.
+
+* OSD paths should follow this convention::
+
+ /var/lib/ceph/osd/<cluster name>-<osd id>
+
+* Preferably, no other mechanisms to mount the volume should exist, and should
+ be removed (like fstab mount points)
+
+The one time process for an existing OSD, with an ID of 0 and using
+a ``"ceph"`` cluster name would look like (the following command will **destroy
+any data** in the OSD)::
+
+ ceph-volume lvm prepare --filestore --osd-id 0 --osd-fsid E3D291C1-E7BF-4984-9794-B60D9FA139CB
+
+The command line tool will not contact the monitor to generate an OSD ID and
+will format the LVM device in addition to storing the metadata on it so that it
+can be started later (for detailed metadata description see
+:ref:`ceph-volume-lvm-tags`).
+
+
+Crush device class
+------------------
+
+To set the crush device class for the OSD, use the ``--crush-device-class`` flag. This will
+work for both bluestore and filestore OSDs::
+
+ ceph-volume lvm prepare --bluestore --data vg/lv --crush-device-class foo
+
+
+.. _ceph-volume-lvm-multipath:
+
+``multipath`` support
+---------------------
+``multipath`` devices are supported if ``lvm`` is configured properly.
+
+**Leave it to LVM**
+
+Most Linux distributions should ship their LVM2 package with
+``multipath_component_detection = 1`` in the default configuration. With this
+setting ``LVM`` ignores any device that is a multipath component and
+``ceph-volume`` will accordingly not touch these devices.
+
+**Using filters**
+
+Should this setting be unavailable, a correct ``filter`` expression must be
+provided in ``lvm.conf``. ``ceph-volume`` must not be able to use both the
+multipath device and its multipath components.
+
+Storing metadata
+----------------
+The following tags will get applied as part of the preparation process
+regardless of the type of volume (journal or data) or OSD objectstore:
+
+* ``cluster_fsid``
+* ``encrypted``
+* ``osd_fsid``
+* ``osd_id``
+* ``crush_device_class``
+
+For :term:`filestore` these tags will be added:
+
+* ``journal_device``
+* ``journal_uuid``
+
+For :term:`bluestore` these tags will be added:
+
+* ``block_device``
+* ``block_uuid``
+* ``db_device``
+* ``db_uuid``
+* ``wal_device``
+* ``wal_uuid``
+
+.. note:: For the complete lvm tag conventions see :ref:`ceph-volume-lvm-tag-api`
+
+
+Summary
+-------
+To recap the ``prepare`` process for :term:`bluestore`:
+
+#. Accepts raw physical devices, partitions on physical devices or logical volumes as arguments.
+#. Creates logical volumes on any raw physical devices.
+#. Generate a UUID for the OSD
+#. Ask the monitor get an OSD ID reusing the generated UUID
+#. OSD data directory is created on a tmpfs mount.
+#. ``block``, ``block.wal``, and ``block.db`` are symlinked if defined.
+#. monmap is fetched for activation
+#. Data directory is populated by ``ceph-osd``
+#. Logical Volumes are assigned all the Ceph metadata using lvm tags
+
+
+And the ``prepare`` process for :term:`filestore`:
+
+#. Accepts raw physical devices, partitions on physical devices or logical volumes as arguments.
+#. Generate a UUID for the OSD
+#. Ask the monitor get an OSD ID reusing the generated UUID
+#. OSD data directory is created and data volume mounted
+#. Journal is symlinked from data volume to journal location
+#. monmap is fetched for activation
+#. devices is mounted and data directory is populated by ``ceph-osd``
+#. data and journal volumes are assigned all the Ceph metadata using lvm tags