File system affinity

You can configure a Ceph File System (CephFS) to prefer a particular Ceph Metadata Server (MDS) over another Ceph MDS.

In some cases you may have two Ceph Metadata Servers, a newer and faster hardware and an older and slower one. You can specify the preference of the newer hardware, by setting the mds_join_fs option, which enforces this file system affinity. Ceph Monitors give preference to MDS standby daemons with mds_join_fs equal to the file system name with the failed rank. The standby-replay daemons are selected before choosing another standby daemon. If no standby daemon exists with the mds_join_fs option, then the Ceph Monitors choose an ordinary standby for replacement or any other available standby as a last resort. The Ceph Monitors periodically examine the Ceph File Systems to see whether a standby with a stronger affinity is available to replace the Ceph MDS that has a lower affinity.