CephFS - cephfs

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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.

CephFS is Ceph’s file system component that provides a robust, fully-featured POSIX-compliant distributed file system. Internally, it maps files to Ceph objects and stores file metadata (for example, file ownership, directory paths, access permissions) in a separate data pool.

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.

A CephFS file system consists of two OSD storage pools, one for the actual data and one for the file metadata.

cephfs driver in LXD

Note

The cephfs driver can only be used for custom storage volumes with content type filesystem.

For other storage volumes, use the Ceph driver. That driver can also be used for custom storage volumes with content type filesystem, but it implements them through Ceph RBD images.

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

You must create the CephFS file system that you want to use beforehand and specify it through the source option.

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.

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.

The cephfs driver in LXD supports snapshots if snapshots are enabled on the server side.

Configuration options

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

Storage pool configuration

Key	Type	Default	Description
cephfs.cluster_name	string	ceph	Name of the Ceph cluster that contains the CephFS file system
cephfs.fscache	bool	false	Enable use of kernel fscache and cachefilesd
cephfs.path	string	/	The base path for the CephFS mount
cephfs.user.name	string	admin	The Ceph user to use
source	string	-	Existing CephFS file system or file system path to use
volatile.pool.pristine	string	true	Whether the CephFS file system 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
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.