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 create.

prepare uses 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. LVM tags identify logical volumes by the role that they play in the Ceph cluster (for example: BlueStore data or BlueStore WAL+DB).

BlueStore is the default backend. Ceph permits changing the backend, which can be done by using the following flags and arguments:

bluestore

Bluestore is the default backend for new OSDs. It offers more flexibility for devices than 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:

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 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}

filestore

Filestore<filestore> is the OSD backend that prepares logical volumes for a 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:

ceph-volume lvm prepare --filestore --data <data block device>

Use this command to deploy filestore with an external journal:

ceph-volume lvm prepare --filestore --data <data block device> --journal <journal block device>

Use this command to enable encryption, and note that the --dmcrypt flag is required:

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:

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:

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 activation 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:

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:

/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.

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"

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 Metadata).

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

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 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. Accepts raw physical devices, partitions on physical devices or logical volumes as arguments.

  2. Creates logical volumes on any raw physical devices.

  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 assigned all the Ceph metadata using lvm tags

And the prepare process for filestore:

  1. Accepts raw physical devices, partitions on physical devices or logical volumes as arguments.

  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