prepare

This subcommand allows a filestore or 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 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 LVM tags.

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

filestore

This is the OSD backend that allows preparation of logical volumes for a 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 prepare --filestore --data data --journal journal

For enabling 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 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 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 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

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:

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 later be startednot 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 later be started (for detailed metadata description see Metadata).

bluestore

The 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 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

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 filestore these tags will be added:

  • journal_device

  • journal_uuid

For 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 Tag API

Summary

To recap the prepare process for bluestore:

  1. Accept a logical volume for block or a raw device (that will get converted to an lv)

  2. Accept partitions or logical volumes for block.wal or block.db

  3. Generate a UUID for the OSD

  4. Ask the monitor get an OSD ID reusing the generated UUID

  5. OSD data directory is created on a tmpfs mount.

  6. block, block.wal, and block.db are symlinked if defined.

  7. monmap is fetched for activation

  8. Data directory is populated by ceph-osd

  9. Logical Volumes are are assigned all the Ceph metadata using lvm tags

And the prepare process for filestore:

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

  2. Generate a UUID for the OSD

  3. Ask the monitor get an OSD ID reusing the generated UUID

  4. OSD data directory is created and data volume mounted

  5. Journal is symlinked from data volume to journal location

  6. monmap is fetched for activation

  7. devices is mounted and data directory is populated by ceph-osd

  8. data and journal volumes are assigned all the Ceph metadata using lvm tags