Ceph RBD - ceph
Description: Ceph is an open-source storage platform that stores its data in a storage cluster based on RADOS. It is highly scalable and, as a distributed system without a single point of failure, very reliable...

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# Ceph RBD - `ceph`

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Ceph is an open-source storage platform that stores its data in a storage cluster based on RADOS. It is highly scalable and, as a distributed system without a single point of failure, very reliable.

Tip

If you want to quickly set up a basic Ceph cluster, check out MicroCeph.

Ceph provides different components for block storage and for file systems.

Ceph RBD is Ceph’s block storage component that distributes data and workload across the Ceph cluster. It uses thin provisioning, which means that it is possible to over-commit resources.

## Terminology

Ceph uses the term object for the data that it stores. The daemon that is responsible for storing and managing data is the Ceph OSD. Ceph’s storage is divided into pools, which are logical partitions for storing objects. They are also referred to as data pools, storage pools or OSD pools.

Ceph block devices are also called RBD images, and you can create snapshots and clones of these RBD images.

## `ceph` driver in LXD

Note

To use the Ceph RBD driver, you must specify it as `ceph`. This is slightly misleading, because it uses only Ceph RBD (block storage) functionality, not full Ceph functionality. For storage volumes with content type `filesystem` (images, containers and custom file-system volumes), the `ceph` driver uses Ceph RBD images with a file system on top (see `block.filesystem`).

Alternatively, you can use the CephFS driver to create storage volumes with content type `filesystem`.

Unlike other storage drivers, this driver does not set up the storage system but assumes that you already have a Ceph cluster installed.

This driver also behaves differently than other drivers in that it provides remote storage. As a result and depending on the internal network, storage access might be a bit slower than for local storage. On the other hand, using remote storage has big advantages in a cluster setup, because all cluster members have access to the same storage pools with the exact same contents, without the need to synchronize storage pools.

The `ceph` driver in LXD uses RBD images for images, and snapshots and clones to create instances and snapshots.

LXD assumes that it has full control over the OSD storage pool. Therefore, you should never maintain any file system entities that are not owned by LXD in a LXD OSD storage pool, because LXD might delete them.

Due to the way copy-on-write works in Ceph RBD, parent RBD images can’t be removed until all children are gone. As a result, LXD automatically renames any objects that are removed but still referenced. Such objects are kept with a `zombie_` prefix until all references are gone and the object can safely be removed.

### Limitations

The `ceph` driver has the following limitations:

Sharing custom volumes between instances

  Custom storage volumes with content type `filesystem` can usually be shared between multiple instances different cluster members. However, because the Ceph RBD driver “simulates” volumes with content type `filesystem` by putting a file system on top of an RBD image, custom storage volumes can only be assigned to a single instance at a time. If you need to share a custom volume with content type `filesystem`, use the CephFS driver instead.

Sharing the OSD storage pool between installations

  Sharing the same OSD storage pool between multiple LXD installations is not supported.

Using an OSD pool of type “erasure”

  To use a Ceph OSD pool of type “erasure”, you must create the OSD pool beforehand. You must also create a separate OSD pool of type “replicated” that will be used for storing metadata. This is required because Ceph RBD does not support `omap`. To specify which pool is “erasure coded”, set the `ceph.osd.data_pool_name` configuration option to the erasure coded pool name and the `source` configuration option to the replicated pool name.

## Configuration options

The following configuration options are available for storage pools that use the `ceph` driver and for storage volumes in these pools.

### Storage pool configuration

`ceph.cluster_name`

Name of the Ceph cluster in which to create new storage pools

Key: | `ceph.cluster_name`
Type: | string
Default: | `ceph`

`ceph.osd.data_pool_name`

Name of the OSD data pool

Key: | `ceph.osd.data_pool_name`
Type: | string

`ceph.osd.pg_num`

Number of placement groups for the OSD storage pool

Key: | `ceph.osd.pg_num`
Type: | string
Default: | `32`

`ceph.osd.pool_name`

Name of the OSD storage pool

Key: | `ceph.osd.pool_name`
Type: | string
Default: | name of the pool

`ceph.rbd.clone_copy`

Whether to use RBD lightweight clones

Key: | `ceph.rbd.clone_copy`
Type: | bool
Default: | `true`

Enable this option to use RBD lightweight clones rather than full dataset copies.

`ceph.rbd.du`

Whether to use RBD `du`

Key: | `ceph.rbd.du`
Type: | bool
Default: | `true`

This option specifies whether to use RBD `du` to obtain disk usage data for stopped instances.

`ceph.rbd.features`

Comma-separated list of RBD features to enable on the volumes

Key: | `ceph.rbd.features`
Type: | string
Default: | `layering`

`ceph.user.name`

The Ceph user to use when creating storage pools and volumes

Key: | `ceph.user.name`
Type: | string
Default: | `admin`

`source`

Existing OSD storage pool to use

Key: | `source`
Type: | string

`volatile.pool.pristine`

Whether the pool was empty on creation time

Key: | `volatile.pool.pristine`
Type: | string
Default: | `true`

Tip

In addition to these configurations, you can also set default values for the storage volume configurations. See Configure default values for storage volumes.

### Storage volume configuration

`block.filesystem`

File system of the storage volume

Key: | `block.filesystem`
Type: | string
Default: | same as `volume.block.filesystem`
Condition: | block-based volume with content type `filesystem`

Valid options are: `btrfs`, `ext4`, `xfs` If not set, `ext4` is assumed.

`block.mount_options`

Mount options for block-backed file system volumes

Key: | `block.mount_options`
Type: | string
Default: | same as `volume.block.mount_options`
Condition: | block-based volume with content type `filesystem`

`security.shifted`

Enable ID shifting overlay

Key: | `security.shifted`
Type: | bool
Default: | same as `volume.security.shifted` or `false`
Condition: | custom volume

Enabling this option allows attaching the volume to multiple isolated instances.

`security.unmapped`

Disable ID mapping for the volume

Key: | `security.unmapped`
Type: | bool
Default: | same as `volume.security.unmappped` or `false`
Condition: | custom volume

`size`

Size/quota of the storage volume

Key: | `size`
Type: | string
Default: | same as `volume.size`
Condition: | appropriate driver

`snapshots.expiry`

When snapshots are to be deleted

Key: | `snapshots.expiry`
Type: | string
Default: | same as `volume.snapshots.expiry`
Condition: | custom volume

Specify an expression like `1M 2H 3d 4w 5m 6y`.

`snapshots.pattern`

Template for the snapshot name

Key: | `snapshots.pattern`
Type: | string
Default: | same as `volume.snapshots.pattern` or `snap%d`
Condition: | custom volume

You can specify a naming template that is used for scheduled snapshots and unnamed snapshots.

The `snapshots.pattern` option takes a Pongo2 template string to format the snapshot name.

To add a time stamp to the snapshot name, use the Pongo2 context variable `creation_date`. Make sure to format the date in your template string to avoid forbidden characters in the snapshot name. For example, set `snapshots.pattern` to `{{ creation_date|date:'2006-01-02_15-04-05' }}` to name the snapshots after their time of creation, down to the precision of a second.

Another way to avoid name collisions is to use the placeholder `%d` in the pattern. For the first snapshot, the placeholder is replaced with `0`. For subsequent snapshots, the existing snapshot names are taken into account to find the highest number at the placeholder’s position. This number is then incremented by one for the new name.

`snapshots.schedule`

Schedule for automatic volume snapshots

Key: | `snapshots.schedule`
Type: | string
Default: | same as `snapshots.schedule`
Condition: | custom volume

Specify either a cron expression (`<minute> <hour> <dom> <month> <dow>`), a comma-separated list of schedule aliases (`@hourly`, `@daily`, `@midnight`, `@weekly`, `@monthly`, `@annually`, `@yearly`), or leave empty to disable automatic snapshots (the default).

`volatile.uuid`

The volume’s UUID

Key: | `volatile.uuid`
Type: | string
Default: | random UUID