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

Key	Type	Default	Description
ceph.cluster_name	string	ceph	Name of the Ceph cluster in which to create new storage pools
ceph.osd.data_pool_name	string	-	Name of the OSD data pool
ceph.osd.pg_num	string	32	Number of placement groups for the OSD storage pool
ceph.osd.pool_name	string	name of the pool	Name of the OSD storage pool
ceph.rbd.clone_copy	bool	true	Whether to use RBD lightweight clones rather than full dataset copies
ceph.rbd.du	bool	true	Whether to use RBD du to obtain disk usage data for stopped instances
ceph.rbd.features	string	layering	Comma-separated list of RBD features to enable on the volumes
ceph.user.name	string	admin	The Ceph user to use when creating storage pools and volumes
source	string	-	Existing OSD storage pool to use
volatile.pool.pristine	string	true	Whether the pool was empty on creation time

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

Key	Type	Condition	Default	Description
block.filesystem	string	block based driver	same as volume.block.filesystem	File system of the storage volume: btrfs, ext4 or xfs (ext4 if not set)
block.mount_options	string	block based driver	same as volume.block.mount_options	Mount options for block devices
security.shifted	bool	custom volume	same as volume.security.shifted or false	Enable ID shifting overlay (allows attach by multiple isolated instances)
security.unmapped	bool	custom volume	same as volume.security.unmapped or false	Disable ID mapping for the volume
size	string	appropriate driver	same as volume.size	Size/quota of the storage volume
snapshots.expiry	string	custom volume	same as volume.snapshots.expiry	Controls when snapshots are to be deleted (expects an expression like 1M 2H 3d 4w 5m 6y)
snapshots.pattern	string	custom volume	same as volume.snapshots.pattern or snap%d	Pongo2 template string that represents the snapshot name (used for scheduled snapshots and unnamed snapshots) [1]
snapshots.schedule	string	custom volume	same as volume.snapshots.schedule	Cron expression (<minute> <hour> <dom> <month> <dow>), a comma-separated list of schedule aliases (@hourly, @daily, @midnight, @weekly, @monthly, @annually, @yearly), or empty to disable automatic snapshots (the default)

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[1]

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.