mon_osd_nearfull_ratio is the maximum / threshold percentage of disk space used before an OSD is considered as a Nearfull OSD. When utilisation crosses Nearfull Ratio, it can reach Backfillfull Ratio and Full Ratio. The default value is 0.85 or 85% of the total available space / capacity.
On croit / IBM / SUSE, once Nearfull Ratio is reached, pools in the cluster are marked read-only until the Nearfull Ratio is increased or more storage space is added or the cluster is balanced in a way that no OSD reaches Nearfull Ratio.
On ceph, pools are read-only when Full Ratio is reached.
It is set on the OSDMap during cluster creation with this parameter:
If the setting is to be changed after cluster creation, one can do so with:
On croit, one can do this by adjusting the sliders at Maintenance → Full Ratios (at the bottom):