CephFS Administrative commands


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>
fs ls
fs rm <filesystem name> [--yes-i-really-mean-it]
fs reset <filesystem name>
fs get <filesystem name>
fs set <filesystem name> <var> <val>
fs add_data_pool <filesystem name> <pool name/id>
fs rm_data_pool <filesystem name> <pool name/id>


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 reducing the number of ranks to 1, setting the cluster_down flag, and then failing the last rank. For example:


ceph fs set <fs_name> max_mds 1 ceph mds deactivate <fs_name>:1 # rank 2 of 2 ceph status # wait for rank 1 to finish stopping ceph fs set <fs_name> cluster_down true ceph mds fail <fs_name>:0

Setting the cluster_down flag prevents standbys from taking over the failed rank.


These commands act on specific mds daemons or ranks.

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.

mds deactivate <role>

Deactivate an MDS, causing it to flush its entire journal to backing RADOS objects and close all open client sessions. Deactivating an MDS is primarily intended for bringing down a rank after reducing the number of active MDS (max_mds). Once the rank is deactivated, the MDS daemon will rejoin the cluster as a standby. <role> can take one of three forms:


Use mds deactivate in conjunction with adjustments to max_mds to shrink an MDS cluster. See Configuring multiple active MDS daemons

tell mds.<daemon name>
mds metadata <gid/name/role>
mds repaired <role>

Global settings

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

“flag name” must be one of [‘enable_multiple’]

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
mds compat rm_incompat
mds compat show
mds getmap
mds set_state
mds rmfailed


The ceph mds set command is the deprecated version of ceph fs set, from before there was more than one filesystem per cluster. It operates on whichever filesystem is marked as the default (see ceph fs set-default.)

mds stat
mds dump  # replaced by "fs get"
mds stop  # replaced by "mds deactivate"
mds set_max_mds # replaced by "fs set max_mds"
mds set # replaced by "fs set"
mds cluster_down  # replaced by "fs set cluster_down"
mds cluster_up  # replaced by "fs set cluster_up"
mds newfs # replaced by "fs new"
mds add_data_pool # replaced by "fs add_data_pool"
mds remove_data_pool #replaced by "fs remove_data_pool"