❓ What?

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.

❔ Why?

The ratio is set as a way to alert and prevent OSDs/disks from reaching Backfillfull Ratio and Full Ratio.

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.

🎤 How?

It is set on the OSDMap during cluster creation with this parameter:

mon_osd_nearfull_ratio = 0.85

If the setting is to be changed after cluster creation, one can do so with:

ceph osd set-nearfull-ratio 0.85

On croit, one can do this by adjusting the sliders at Maintenance → Full Ratios (at the bottom):

👓 References