# Terminology¶

A Ceph cluster may have zero or more CephFS file systems. CephFS file systems have a human readable name (set in 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, one by default, which start at zero. A rank may be thought of as a metadata shard. Controlling the number of ranks in a file system is described in Configuring multiple active MDS daemons

Each CephFS ceph-mds process (a daemon) initially starts up without a rank. It may be assigned one by the monitor cluster. A daemon may only hold one rank at a time. Daemons only give up a rank when the ceph-mds process stops.

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

A daemon has a name that is set statically by the administrator when the daemon is first configured. Typical configurations use the hostname where the daemon runs as the daemon name.

A ceph-mds daemons can be assigned to a particular file system by setting the mds_join_fs configuration option to the file system name.

Each time a daemon starts up, it is also assigned a GID, which is unique to this particular process lifetime of the daemon. The GID is an integer.

# Referring to MDS daemons¶

Most of the administrative commands that refer to an MDS daemon accept a flexible argument format that may contain a rank, a GID or a name.

Where a rank is used, this may optionally be qualified with a leading file system name or ID. 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, if we had an MDS daemon which was called ‘myhost’, had GID 5446, and was assigned rank 0 in the file system ‘myfs’ which had FSCID 3, then any of the following would be 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 monitor, the monitor will wait mds_beacon_grace seconds (default 15 seconds) before marking the daemon as laggy. If a standby is available, the monitor will immediately replace the laggy daemon.

Each file system may specify a number of standby daemons to be considered healthy. This number includes daemons in standby-replay waiting for a rank to fail (remember that a standby-replay daemon will not be assigned to take over a failure for another rank or a failure in a another CephFS file system). The pool of standby daemons not in replay count towards any file system count. Each file system may set the number of standby daemons wanted using:

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 to reduce failover time in the event the active MDS becomes unavailable. Each active MDS may have only one standby-replay daemon following it.

Configuring standby-replay on a file system is done using:

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 may want to have an MDS used for a particular file system. Or, perhaps you have larger MDSs on better hardware that should be preferred over a last-resort standby on lesser or over-provisioned hardware. To express this preference, CephFS provides a configuration option for MDS called mds_join_fs which enforces this affinity.

As part of any failover, the Ceph 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 a vanilla standby (no setting for mds_join_fs) for the replacement or any other available standby as a last resort. Note, this does not change the behavior that standby-replay daemons are always selected before looking at other standbys.

Even further, the monitors will regularly examine the CephFS file systems 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:

$ceph fs dump dumped fsmap epoch 399 ... Filesystem 'cephfs' (27) ... e399 max_mds 1 in 0 up {0=20384} failed damaged stopped ... [mds.a{0:20384} state up:active seq 239 addr [v2:127.0.0.1:6854/966242805,v1:127.0.0.1:6855/966242805]] Standby daemons: [mds.b{-1:10420} state up:standby seq 2 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]] You may set mds_join_fs on the standby to enforce your preference:$ ceph config set mds.b mds_join_fs cephfs

after automatic failover:

\$ ceph fs dump
dumped fsmap epoch 405
e405
...
Filesystem 'cephfs' (27)
...
max_mds 1
in      0
up      {0=10420}
failed
damaged
stopped
...
[mds.b{0:10420} state up:active seq 274 join_fscid=27 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]]

Standby daemons:

[mds.a{-1:10720} state up:standby seq 2 addr [v2:127.0.0.1:6854/1340357658,v1:127.0.0.1:6855/1340357658]]

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.