CephFS Administrative commands



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

These commands operate on the CephFS filesystems in your Ceph cluster. Note that by default only one filesystem is permitted: to enable creation of multiple filesystems use ceph fs flag set enable_multiple true.

fs new <filesystem name> <metadata pool name> <data pool name>

This command creates a new file system. The file system name and metadata pool name are self-explanatory. 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.

fs ls

List all file systems by name.

fs dump [epoch]

This dumps the FSMap at the given epoch (default: current) which includes all file system settings, MDS daemons and the ranks they hold, and the list of standby MDS daemons.

fs rm <filesystem name> [--yes-i-really-mean-it]

Destroy a CephFS file system. This wipes information about the state of the file system from the FSMap. The metadata pool and data pools are untouched and must be destroyed separately.

fs get <filesystem name>

Get information about the named file system, including settings and ranks. This is a subset of the same information from the fs dump command.

fs set <filesystem name> <var> <val>

Change a setting on a file system. These settings are specific to the named file system and do not affect other file systems.

fs add_data_pool <filesystem name> <pool name/id>

Add a data pool to the file system. This pool can be used for file layouts as an alternate location to store file data.

fs rm_data_pool <filesystem name> <pool name/id>

This command removes the specified pool from the list of data pools for the file system. If any files have layouts for the removed data pool, the file data will become unavailable. The default data pool (when creating the file system) cannot be removed.


fs set <fs name> max_file_size <size in bytes>

CephFS has a configurable maximum file size, and it’s 1TB by default. You may wish to set this limit higher if you expect to store large files in CephFS. It is a 64-bit field.

Setting max_file_size to 0 does not disable the limit. It would simply limit clients to only creating empty files.

Maximum file sizes and performance

CephFS enforces the maximum file size limit at the point of appending to files or setting their size. It does not affect how anything is stored.

When users create a file of an enormous size (without necessarily writing any data to it), some operations (such as deletes) cause the MDS to have to do a large number of operations to check if any of the RADOS objects within the range that could exist (according to the file size) really existed.

The max_file_size setting prevents users from creating files that appear to be eg. exabytes in size, causing load on the MDS as it tries to enumerate the objects during operations like stats or deletes.

Taking the cluster down

Taking a CephFS cluster down is done by setting the down flag:

fs set <fs_name> down true

To bring the cluster back online:

fs set <fs_name> down false

This will also restore the previous value of max_mds. MDS daemons are brought down in a way such that journals are flushed to the metadata pool and all client I/O is stopped.

Taking the cluster down rapidly for deletion or disaster recovery

To allow rapidly deleting a file system (for testing) or to quickly bring the file system and MDS daemons down, use the fs fail command:

fs fail <fs_name>

This command sets a file system flag to prevent standbys from activating on the file system (the joinable flag).

This process can also be done manually by doing the following:

fs set <fs_name> joinable false

Then the operator can fail all of the ranks which causes the MDS daemons to respawn as standbys. The file system will be left in a degraded state.

# For all ranks, 0-N:
mds fail <fs_name>:<n>

Once all ranks are inactive, the file system may also be deleted or left in this state for other purposes (perhaps disaster recovery).

To bring the cluster back up, simply set the joinable flag:

fs set <fs_name> joinable true


Most commands manipulating MDSs take a <role> argument which can take one of three forms:


Commands to manipulate MDS daemons:

mds fail <gid/name/role>

Mark an MDS daemon as failed. This is equivalent to what the cluster would do if an MDS daemon had failed to send a message to the mon for mds_beacon_grace second. If the daemon was active and a suitable standby is available, using mds fail will force a failover to the standby.

If the MDS daemon was in reality still running, then using mds fail will cause the daemon to restart. If it was active and a standby was available, then the “failed” daemon will return as a standby.

tell mds.<daemon name> command ...

Send a command to the MDS daemon(s). Use mds.* to send a command to all daemons. Use ceph tell mds.* help to learn available commands.

mds metadata <gid/name/role>

Get metadata about the given MDS known to the Monitors.

mds repaired <role>

Mark the file system rank as repaired. Unlike the name suggests, this command does not change a MDS; it manipulates the file system rank which has been marked damaged.

Minimum Client Version

It is sometimes desirable to set the minimum version of Ceph that a client must be running to connect to a CephFS cluster. Older clients may sometimes still be running with bugs that can cause locking issues between clients (due to capability release). CephFS provides a mechanism to set the minimum client version:

fs set <fs name> min_compat_client <release>

For example, to only allow Nautilus clients, use:

fs set cephfs min_compat_client nautilus

Clients running an older version will be automatically evicted.

Global settings

fs flag set <flag name> <flag val> [<confirmation string>]

Sets a global CephFS flag (i.e. not specific to a particular file system). Currently, the only flag setting is ‘enable_multiple’ which allows having multiple CephFS file systems.

Some flags require you to confirm your intentions with “–yes-i-really-mean-it” or a similar string they will prompt you with. Consider these actions carefully before proceeding; they are placed on especially dangerous activities.


These commands are not required in normal operation, and exist for use in exceptional circumstances. Incorrect use of these commands may cause serious problems, such as an inaccessible filesystem.

mds compat rm_compat

Removes an compatibility feature flag.

mds compat rm_incompat

Removes an incompatibility feature flag.

mds compat show

Show MDS compatibility flags.

mds rmfailed

This removes a rank from the failed set.

fs reset <filesystem name>

This command resets the file system state to defaults, except for the name and pools. Non-zero ranks are saved in the stopped set.