This document is for a development version of Ceph.


A Ceph cluster may have zero or more CephFS file systems. Each CephFS has a human readable name (set at creation time with fs new) and an integer ID. The ID is called the file system cluster ID, or FSCID.

Each CephFS file system has a number of ranks, numbered beginning with zero. By default there is one rank per file system. A rank may be thought of as a metadata shard. Management of ranks is described in Configuring multiple active MDS daemons .

Each CephFS ceph-mds daemon starts without a rank. It may be assigned one by the cluster’s monitors. A daemon may only hold one rank at a time, and only give up a rank when the ceph-mds process stops.

If a rank is not associated with any daemon, that rank is considered failed. Once a rank is assigned to a daemon, the rank is considered up.

Each ceph-mds daemon has a name that is assigned statically by the administrator when the daemon is first configured. Each daemon’s name is typically that of the hostname where the process runs.

A ceph-mds daemon may be assigned to a specific file system by setting its mds_join_fs configuration option to the file system’s name.

When a ceph-mds daemon starts, it is also assigned an integer GID, which is unique to this current daemon’s process. In other words, when a ceph-mds daemon is restarted, it runs as a new process and is assigned a new GID that is different from that of the previous process.

Referring to MDS daemons

Most administrative commands that refer to a ceph-mds daemon (MDS) accept a flexible argument format that may specify a rank, a GID or a name.

Where a rank is used, it may optionally be qualified by a leading file system name or GID. If a daemon is a standby (i.e. it is not currently assigned a rank), then it may only be referred to by GID or name.

For example, say we have an MDS daemon with name ‘myhost’ and GID 5446, and which is assigned rank 0 for the file system ‘myfs’ with FSCID 3. Any of the following are suitable forms of the fail command:

ceph mds fail 5446     # GID
ceph mds fail myhost   # Daemon name
ceph mds fail 0        # Unqualified rank
ceph mds fail 3:0      # FSCID and rank
ceph mds fail myfs:0   # File System name and rank

Managing failover

If an MDS daemon stops communicating with the cluster’s monitors, the monitors will wait mds_beacon_grace seconds (default 15) before marking the daemon as laggy. If a standby MDS is available, the monitor will immediately replace the laggy daemon.

Each file system may specify a minimum number of standby daemons in order to be considered healthy. This number includes daemons in the standby-replay state waiting for a rank to fail. (Note, the monitors will not assign a standby-replay daemon to take over a failure for another rank or a failure in a different CephFS file system). The pool of standby daemons not in replay counts towards any file system count. Each file system may set the desired number of standby daemons by:

ceph fs set <fs name> standby_count_wanted <count>

Setting count to 0 will disable the health check.

Configuring standby-replay

Each CephFS file system may be configured to add standby-replay daemons. These standby daemons follow the active MDS’s metadata journal in order to reduce failover time in the event that the active MDS becomes unavailable. Each active MDS may have only one standby-replay daemon following it.

Configuration of standby-replay on a file system is done using the below:

ceph fs set <fs name> allow_standby_replay <bool>

Once set, the monitors will assign available standby daemons to follow the active MDSs in that file system.

Once an MDS has entered the standby-replay state, it will only be used as a standby for the rank that it is following. If another rank fails, this standby-replay daemon will not be used as a replacement, even if no other standbys are available. For this reason, it is advised that if standby-replay is used then every active MDS should have a standby-replay daemon.

Configuring MDS file system affinity

You might elect to dedicate an MDS to a particular file system. Or, perhaps you have MDSs that run on better hardware that should be preferred over a last-resort standby on modest or over-provisioned systems. To configure this preference, CephFS provides a configuration option for MDS called mds_join_fs which enforces this affinity.

When failing over MDS daemons, a cluster’s monitors will prefer standby daemons with mds_join_fs equal to the file system name with the failed rank. If no standby exists with mds_join_fs equal to the file system name, it will choose an unqualified standby (no setting for mds_join_fs) for the replacement. As a last resort, a standby for another filesystem will be chosen, although this behavior can be disabled:

ceph fs set <fs name> refuse_standby_for_another_fs true

Note, configuring MDS file system affinity does not change the behavior that standby-replay daemons are always selected before other standbys.

Even further, the monitors will regularly examine the CephFS file systems even when stable to check if a standby with stronger affinity is available to replace an MDS with lower affinity. This process is also done for standby-replay daemons: if a regular standby has stronger affinity than the standby-replay MDS, it will replace the standby-replay MDS.

For example, given this stable and healthy file system:

$ ceph fs dump
dumped fsmap epoch 399
Filesystem 'cephfs' (27)
max_mds 1
in      0
up      {0=20384}
[mds.a{0:20384} state up:active seq 239 addr [v2:,v1:]]

Standby daemons:

[mds.b{-1:10420} state up:standby seq 2 addr [v2:,v1:]]

You may set mds_join_fs on the standby to enforce your preference:

$ ceph config set mds.b mds_join_fs cephfs

after automatic failover:

$ ceph fs dump
dumped fsmap epoch 405
Filesystem 'cephfs' (27)
max_mds 1
in      0
up      {0=10420}
[mds.b{0:10420} state up:active seq 274 join_fscid=27 addr [v2:,v1:]]

Standby daemons:

[mds.a{-1:10720} state up:standby seq 2 addr [v2:,v1:]]

Note in the above example that mds.b now has join_fscid=27. In this output, the file system name from mds_join_fs is changed to the file system identifier (27). If the file system is recreated with the same name, the standby will follow the new file system as expected.

Finally, if the file system is degraded or undersized, no failover will occur to enforce mds_join_fs.

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