Create a Ceph file system

Creating pools

A Ceph file system requires at least two RADOS pools, one for data and one for metadata. There are important considerations when planning these pools:

  • We recommend configuring at least 3 replicas for the metadata pool, as data loss in this pool can render the entire file system inaccessible. Configuring 4 would not be extreme, especially since the metadata pool’s capacity requirements are quite modest.

  • We recommend the fastest feasible low-latency storage devices (NVMe, Optane, or at the very least SAS/SATA SSD) for the metadata pool, as this will directly affect the latency of client file system operations.

  • We strongly suggest that the CephFS metadata pool be provisioned on dedicated SSD / NVMe OSDs. This ensures that high client workload does not adversely impact metadata operations. See Device classes to configure pools this way.

  • The data pool used to create the file system is the “default” data pool and the location for storing all inode backtrace information, which is used for hard link management and disaster recovery. For this reason, all CephFS inodes have at least one object in the default data pool. If erasure-coded pools are planned for file system data, it is best to configure the default as a replicated pool to improve small-object write and read performance when updating backtraces. Separately, another erasure-coded data pool can be added (see also Erasure code) that can be used on an entire hierarchy of directories and files (see also File layouts).

Refer to Pools to learn more about managing pools. For example, to create two pools with default settings for use with a file system, you might run the following commands:

$ ceph osd pool create cephfs_data
$ ceph osd pool create cephfs_metadata

The metadata pool will typically hold at most a few gigabytes of data. For this reason, a smaller PG count is usually recommended. 64 or 128 is commonly used in practice for large clusters.


The names of the file systems, metadata pools, and data pools can only have characters in the set [a-zA-Z0-9_-.].

Creating a file system

Once the pools are created, you may enable the file system using the fs new command:

$ ceph fs new <fs_name> <metadata> <data> [--force] [--allow-dangerous-metadata-overlay] [<fscid:int>] [--recover]

This command creates a new file system with specified metadata and data pool. The specified data pool is the default data pool and cannot be changed once set. Each file system has its own set of MDS daemons assigned to ranks so ensure that you have sufficient standby daemons available to accommodate the new file system.

The --force option is used to achieve any of the following:

  • To set an erasure-coded pool for the default data pool. Use of an EC pool for the default data pool is discouraged. Refer to Creating pools for details.

  • To set non-empty pool (pool already contains some objects) for the metadata pool.

  • To create a file system with a specific file system’s ID (fscid). The --force option is required with --fscid option.

The --allow-dangerous-metadata-overlay option permits the reuse metadata and data pools if it is already in-use. This should only be done in emergencies and after careful reading of the documentation.

If the --fscid option is provided then this creates a file system with a specific fscid. This can be used when an application expects the file system’s ID to be stable after it has been recovered, e.g., after monitor databases are lost and rebuilt. Consequently, file system IDs don’t always keep increasing with newer file systems.

The --recover option sets the state of file system’s rank 0 to existing but failed. So when a MDS daemon eventually picks up rank 0, the daemon reads the existing in-RADOS metadata and doesn’t overwrite it. The flag also prevents the standby MDS daemons to join the file system.

For example:

$ ceph fs new cephfs cephfs_metadata cephfs_data
$ ceph fs ls
name: cephfs, metadata pool: cephfs_metadata, data pools: [cephfs_data ]

Once a file system has been created, your MDS(s) will be able to enter an active state. For example, in a single MDS system:

$ ceph mds stat
cephfs-1/1/1 up {0=a=up:active}

Once the file system is created and the MDS is active, you are ready to mount the file system. If you have created more than one file system, you will choose which to use when mounting.

If you have created more than one file system, and a client does not specify a file system when mounting, you can control which file system they will see by using the ceph fs set-default command.

Adding a Data Pool to the File System

See Adding a data pool to the File System.

Using Erasure Coded pools with CephFS

You may use Erasure Coded pools as CephFS data pools as long as they have overwrites enabled, which is done as follows:

ceph osd pool set my_ec_pool allow_ec_overwrites true

Note that EC overwrites are only supported when using OSDs with the BlueStore backend.

You may not use Erasure Coded pools as CephFS metadata pools, because CephFS metadata is stored using RADOS OMAP data structures, which EC pools cannot store.