Ceph: How to failover an MDS daemon

Issue

How to failover an MDS daemon

In a Ceph cluster with more than one MDS daemon running, to promote a standby MDS to active, it is necessary to fail one of the current active MDS daemons.

Some examples:

[user@edon-0 ~]# ceph fs status
root - 88 clients
====
RANK  STATE             MDS                ACTIVITY     DNS    INOS   DIRS   CAPS  
 0    active  root.edon-1.amvgfe  Reqs:    8 /s  76.1k  75.0k  1501   38.9k  
 1    active  root.edon-3.wnboxv  Reqs:  121 /s  5321k  5309k  12.1k   494k  
 2    active  root.edon-7.oqqvka  Reqs:    0 /s  5370   3575    289   3710   
 3    active  root.edon-9.qvfdrs  Reqs:    1 /s  8184k  8178k  84.2k   434k  
 4    active  root.edon-5.lbrqru  Reqs:  556 /s   319k   307k  23.2k  45.9k  
    POOL       TYPE     USED  AVAIL  
cephfs.meta  metadata  46.2G  12.8T  
cephfs.data    data     124T   246T  
      STANDBY MDS        
root.edon-2.rdnzhn  
root.edon-6.kfqjor  
root.edon-4.pjrzki  
MDS version: ceph version 17.2.6-100.el9cp (ea4e3ef8df2cf26540aae06479df031dcfc80343) quincy (stable)

[user@edon0 ~]$ sudo cephadm shell -- ceph fs status
cephfs - 207 clients
======
RANK  STATE                MDS             ACTIVITY     DNS    INOS   DIRS   CAPS
0    active  cephfs.edon1.bnsdfe  Reqs: 1804 /s  17.9M  17.7M  6856k  1811k
1    active  cephfs.edon3.ipwxcw  Reqs:   57 /s   942    301   1063     67
      POOL         TYPE     USED  AVAIL
cephfs_metadata  metadata  40.9G   117T
  cephfs_data      data    39.8T   117T
         STANDBY MDS
cephfs.edon2.gruurg
MDS version: ceph version 16.2.10-187.el8cp (5d6355e2bccd18b5c6457a34cb666d773f21823d) pacific (stable)