ceph/doc/ceph-volume/lvm/prepare.rst

.. _ceph-volume-lvm-prepare:

``prepare``
===========
This subcommand allows a :term:`filestore` or :term:`bluestore` setup. It is
recommended to pre-provision a logical volume before using it with
``ceph-volume lvm``.

Logical volumes are not altered except for adding extra metadata.

.. note:: This is part of a two step process to deploy an OSD. If looking for
          a single-call way, please see :ref:`ceph-volume-lvm-create`

To help identify volumes, the process of preparing a volume (or volumes) to
work with Ceph, the tool will assign a few pieces of metadata information using
:term:`LVM tags`.

:term:`LVM tags` makes volumes easy to discover later, and help identify them as
part of a Ceph system, and what role they have (journal, filestore, bluestore,
etc...)

Although initially :term:`filestore` is supported (and supported by default)
the back end can be specified with:


* :ref:`--filestore <ceph-volume-lvm-prepare_filestore>`
* :ref:`--bluestore <ceph-volume-lvm-prepare_bluestore>`

.. _ceph-volume-lvm-prepare_filestore:

``filestore``
-------------
This is the OSD backend that allows preparation of logical volumes for
a :term:`filestore` objectstore OSD.

It can use a logical volume for the OSD data and a partitioned physical device
or logical volume for the journal.  No special preparation is needed for these
volumes other than following the minimum size requirements for data and
journal.

The API call looks like::

    ceph-volume lvm prepare --filestore --data volume_group/lv_name --journal journal

For enabling :ref:`encryption <ceph-volume-lvm-encryption>`, the ``--dmcrypt`` flag is required::

    ceph-volume lvm prepare --filestore --dmcrypt --data volume_group/lv_name --journal journal

There is flexibility to use a raw device or partition as well for ``--data``
that will be converted to a logical volume. This is not ideal in all situations
since ``ceph-volume`` is just going to create a unique volume group and
a logical volume from that device.

When using logical volumes for ``--data``, the  value *must* be a volume group
name and a logical volume name separated by a ``/``. Since logical volume names
are not enforced for uniqueness, this prevents using the wrong volume. The
``--journal`` can be either a logical volume *or* a partition.

When using a partition, it *must* contain a ``PARTUUID`` discoverable by
``blkid``, so that it can later be identified correctly regardless of the
device name (or path).

When using a partition, this is how it would look for ``/dev/sdc1``::

    ceph-volume lvm prepare --filestore --data volume_group/lv_name --journal /dev/sdc1

For a logical volume, just like for ``--data``, a volume group and logical
volume name are required::

    ceph-volume lvm prepare --filestore --data volume_group/lv_name --journal volume_group/journal_lv

A generated uuid is used to ask the cluster for a new OSD. These two pieces are
crucial for identifying an OSD and will later be used throughout the
:ref:`ceph-volume-lvm-activate` process.

The OSD data directory is created using the following convention::

    /var/lib/ceph/osd/<cluster name>-<osd id>

At this point the data volume is mounted at this location, and the journal
volume is linked::

      ln -s /path/to/journal /var/lib/ceph/osd/<cluster_name>-<osd-id>/journal

The monmap is fetched using the bootstrap key from the OSD::

      /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

``ceph-osd`` will be called to populate the OSD directory, that is already
mounted, re-using all the pieces of information from the initial steps::

      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


.. _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`).


.. _ceph-volume-lvm-prepare_bluestore:

``bluestore``
-------------
The :term:`bluestore` objectstore is the default for new OSDs. It offers a bit
more flexibility for devices. Bluestore supports the following configurations:

* A block device, a block.wal, 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

It can accept a whole device (or partition), or a logical volume for ``block``.
If a physical device is provided it will then be turned into a logical volume.
This allows a simpler approach at using LVM but at the cost of flexibility:
there are no options or configurations to change how the LV is created.

The ``block`` is specified with the ``--data`` flag, and in its simplest use
case it looks like::

    ceph-volume lvm prepare --bluestore --data vg/lv

A raw device can be specified in the same way::

    ceph-volume lvm prepare --bluestore --data /path/to/device

For enabling :ref:`encryption <ceph-volume-lvm-encryption>`, the ``--dmcrypt`` flag is required::

    ceph-volume lvm prepare --bluestore --dmcrypt --data vg/lv

If a ``block.db`` or a ``block.wal`` is needed (they are optional for
bluestore) they can be specified with ``--block.db`` and ``--block.wal``
accordingly. These can be a physical device (they **must** be a partition) or
a logical volume.

For both ``block.db`` and ``block.wal`` partitions aren't made logical volumes
because they can be used as-is. Logical Volumes are also allowed.

While creating the OSD directory, the process will use a ``tmpfs`` mount to
place all the files needed for the OSD. These files are initially created by
``ceph-osd --mkfs`` and are fully ephemeral.

A symlink is always created for the ``block`` device, and optionally for
``block.db`` and ``block.wal``. For a cluster with a default name, and an OSD
id of 0, the directory could look like::

    # 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`` create
a volume group and a logical volume using the following convention:

* volume group name: ``ceph-{cluster fsid}`` or if the vg exists already
  ``ceph-{random uuid}``

* logical volume name: ``osd-block-{osd_fsid}``


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
---------------------
Devices that come from ``multipath`` are not supported as-is. The tool will
refuse to consume a raw multipath device and will report a message like::

    -->  RuntimeError: Cannot use device (/dev/mapper/<name>). A vg/lv path or an existing device is needed

The reason for not supporting multipath is that depending on the type of the
multipath setup, if using an active/passive array as the underlying physical
devices, filters are required in ``lvm.conf`` to exclude the disks that are part of
those underlying devices.

It is unfeasible for ceph-volume to understand what type of configuration is
needed for LVM to be able to work in various different multipath scenarios. The
functionality to create the LV for you is merely a (naive) convenience,
anything that involves different settings or configuration must be provided by
a config management system which can then provide VGs and LVs for ceph-volume
to consume.

This situation will only arise when trying to use the ceph-volume functionality
that creates a volume group and logical volume from a device. If a multipath
device is already a logical volume it *should* work, given that the LVM
configuration is done correctly to avoid issues.


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`:

#. Accept a logical volume for block or a raw device (that will get converted
   to an lv)
#. Accept partitions or logical volumes for ``block.wal`` or ``block.db``
#. 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`:

#. Accept only logical volumes for data and journal (both required)
#. 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