The process of removing OSDs from a Ceph cluster involves removing all the entries of these OSDs from cluster maps.

Remove the OSDs from the CRUSH map. For this, log in to any of the cluster nodes and perform the following commands:

# ceph osd crush remove osd.9
# ceph osd crush remove osd.10
# ceph osd crush remove osd.11

As soon as OSDs are removed from the CRUSH map, the Ceph cluster becomes healthy. You should also observe the OSD map; since we have not removed the OSDs, it will show as 12 OSD, 9 UP, 9 IN:

Remove the OSD authentication key:

# ceph auth del osd.9
# ceph auth del osd.10
# ceph auth del osd.11

Finally, remove the OSD and check your cluster status. You should see 9 OSD, 9 UP, 9 IN, and the cluster health should be OK:

To keep your cluster clean, perform some housekeeping. As we have removed all the OSDs from the CRUSH map, ceph-node4 does not hold any item. Remove ceph-node4 from the CRUSH map to remove all traces of this node from the Ceph cluster:

# ceph osd crush remove ceph-node4