Terminology

A Ceph cluster may have zero or more CephFS file systems. CephFSfile 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 describedin Configuring multiple active MDS daemons

Each CephFS ceph-mds process (a daemon) initially starts upwithout a rank. It may be assigned one by the monitor cluster.A daemon may only hold one rank at a time. Daemons only give upa rank when the ceph-mds process stops.

If a rank is not associated with a daemon, the rank isconsidered 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 administratorwhen the daemon is first configured. Typical configurationsuse the hostname where the daemon runs as the daemon name.

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